Improvement of the Tribological Properties of Alumina Coatings by Zirconia Addition

Al2O3 and Al2O3-ZrO2 coatings were deposited on stainless steel using atmospheric plasma spraying. The influence of arc current and zirconia addition on the surface morphology of the coating, phase composition and tribological properties under dry sliding conditions were investigated. The addition of zirconia reduced the surface roughness of the coatings. The X-ray diffraction measurements indicated that the Al2O3 coatings were composed of β-Al2O3, α-Al2O3, and γ-Al2O3 phases. The addition of zirconia led to the formation of tetragonal and monoclinic phases of zirconia in the as-sprayed coatings. The friction coefficients of Al2O3 and Al2O3-ZrO2 coatings were similar and varied in the range of 0.72–0.75. The specific wear rates of the as-sprayed coatings were reduced with the increase of arc current. It was obtained that the wear rates of the Al2O3-ZrO2 coatings were at least three times lower compared to Al2O3 coatings.


Introduction
Aluminum oxide coatings have received wide attention because of their good mechanical and tribological properties, high resistance to thermal degradation and corrosion and a low electrical conductivity. These excellent properties allow alumina to be used in various fields like wear or corrosion protection of metal surfaces [1][2][3][4][5][6]. However, despite their high hardness, the alumina coatings are brittle and have relatively high friction coefficients under dry sliding conditions. In order to improve the toughness, brittleness, thermal or tribological properties of alumina coatings, various additives such as ZrO 2 [7][8][9][10], TiO 2 [7,11], graphite [12], and graphene nanoplatelets [13] are used.
Atmospheric plasma spraying is one of the most popular thermal spray processes used for the deposition of alumina and alumina composite coatings. It was demonstrated that the final properties of alumina coatings are strongly related to the phase composition as well as the amount of additives [2][3][4][5][8][9][10]12,14,15]. The variation of the spraying parameters (power, distance, substrate temperature, feedstock powder injection place) and feedstock powders characteristics (size, shape, composition etc.) strongly affects the phase composition and properties of the sprayed coatings [2][3][4][5][8][9][10]14,16]. It was demonstrated that the presence of γ-Al 2 O 3 phase enhanced the thermal and electrical properties. Meanwhile, the αAl 2 O 3 phase could increase the hardness and dielectric properties of the alumina or alumina composite coatings [1,2,15]. Deng et al. [1] demonstrated that the friction coefficient and wear rate strongly depend on the counterpart body type used. The coefficient of

Materials and Methods
The substrates used for the deposition of coatings were made from AISI 304L steel (40 mm × 10 mm × 1.5 mm). The plasma torch used for the deposition of coatings was designed and built at the Lithuanian Energy Institute (Kaunas, Lithuania) [12]. The coatings were sprayed using an air and hydrogen gas mixture. A total air flow rate of 4.72 and 4.70 g/s was used for the plasma jet formation and feedstock powder injection when Al 2 O 3 and Al 2 O 3 -10 wt.% ZrO 2 coatings were deposited, respectively. An air flow rate of 0.60 g/s was used for the feedstock powder injection. Additionally, hydrogen gas with the flow rate of~0.1 g/s was used. The hydrogen gas was injected inside the reactor nozzle. The addition of the hydrogen was used in order to enhance the plasma temperature and increase the melting degree of feedstock powders. Al 2 O 3 (non-regular shape, size range from 63 to 140 µm, 98.5% purity) and ZrO 2 -8%Y 2 O 3 (type ZRO-113/114, 99% purity, spherical shape, purchased from PRAXAIR Surface Technologies, Indianapolis, IN, USA) powders were used to spray the coatings. The mixture of alumina and zirconia powders of Al 2 O 3 -10 wt.% ZrO 2 was prepared. The feedstock powders were mechanically mixed for 24 h and dried before the deposition. The steel samples were placed on water cooled holder in the distance of 70 mm from the plasma torch nozzle. The plasma torch arc currents during the experiments were 180 and 220 A. The plasma torch was moving forward and backward during the spraying. The deposition duration was~40 s. The detailed plasma spraying parameters are given in Table 2. More detailed information on the experimental setup and the methodology for calculations of average plasma velocity and temperature at the nozzle outlet are found in [12]. A S-3400N scanning electron microscope (SEM, Hitachi, Tokyo, Japan) and a portable Surftest SJ-210 Series surface roughness tester (Version 2.00 with standard ISO 1997 Mitutoyo, Kawasaki, Japan) were used for the surface morphology and roughness analysis of the coatings, respectively. Linear roughness of the coatings was measured and the length of one profile measurement was 4 mm. Three samples of each type of coating type were measured and at least four measurements of each sample were done in order to calculate average linear roughness values. Error bars represent the standard deviation values. The energy dispersive X-ray spectroscopy (EDX) method (Bruker Quad 5040 spectrometer, AXS Microanalysis GmbH, Billerica, MA, USA) was used for determination of elemental composition of sprayed coatings. The EDX measurements were done for a surface area of~1.15 mm 2 (magnification was 100×) at 5 different points using 15 kV acceleration voltage. The structure of the coatings was analyzed using X-ray diffraction (XRD) (Bruker D8 Discover, Billerica, MA, USA) with a standard Bragg-Brentano focusing geometry in a 5 • -80 • range using the CuKα (λ = 0.154059 nm) radiation. A ball-on-flat configuration on a tribometer (UMT-2, CETR, Campbell, CA, USA) was used for measuring the tribological properties of prepared coatings and the initial steel substrate. A sliding velocity of 0.05 m/s for 50 min (distance 150 m) with a constant normal load of 0.8 N was used for the tests. The length of stroke in the tribological test was 5 mm. The tribological tests on the alumina coatings were done three times for two samples of each series. Meanwhile, two samples for each series of alumina-zirconia coatings were tested at two different surface areas. The mean friction coefficient was calculated using the last 10% of the data values [12]. All tribological tests were performed in dry-sliding conditions at 21 • C with a relative humidity RH = 20 ± 5%. As the counterpart a 10 mm diameter Al 2 O 3 ball (purity 99.5% and grade 10) was used. A 3D white-light optical interferometer (Counter GT-K0, Bruker, Billerica, MA, USA), with the use of the Vision64 software were used for the examination of the amount of coating removed during the tribological tests.

Results and Discussions
SEM images of the alumina and alumina-zirconia coatings deposited at various arc currents are given in Figure 1. It can be seen that for the aluminum oxide coatings, the amount of molten particles had increased, and the size of particles had slightly reduced at the higher torch power (Figure 1a-d). Spherical and irregular shaped particles of size from 1 to 20 µm could be found. The Al 2 O 3 coatings had some pores, voids and micro-cracks (Figure 1b,d). The surfaces of the Al 2 O 3 -ZrO 2 coatings are slightly more homogenous and smoother than the Al 2 O 3 coatings. The surface contains partially and fully molten particle zones, spheroidized particles, and also, some voids and pores could be observed (Figure 1f,h). The thickness of the alumina coatings was 30-40 µm. The thicknesses of the Al 2 O 3 -ZrO 2 coatings were up to 40 and~70 µm prepared at 180 A and at 220 A, respectively. With increasing arc current, it could be seen that the melting degree of the particles had enhanced as the average plasma temperature had increased by~70-85 K. It should be noted that the plasma jet temperature was higher than the melting points of alumina (~2320 K) and zirconia (~2950 K) feedstock powders [8]. With the enhancement in arc current from 180 to 220 A, globule-like formations could be perceived, and cauliflower-like structures could be ascertained (Figure 1h). The small amount of unmelted or semi-molten particle fragments obtained on the surface of the coatings prepared at the higher arc current is due to the lower temperature in the peripheral region of the plasma jet and the different sizes of feedstock powders used [15]. higher arc current is due to the lower temperature in the peripheral region of the plasma jet and the different sizes of feedstock powders used [15]. The distribution of the oxygen, zirconium, aluminum, and yttrium on the surface of the Al2O3-ZrO2 coatings are presented in Figure 2. The images indicated that the alumina and zirconia feedstock powders were homogenously spread on the surface. The EDX   The used Al2O3 and ZrO2 feedstock powders were mixed mechanically. Thus, due to different size and shape of used Al2O3 (non-regular shape) and ZrO2 (mainly spherical shape) powders and their densities, the flowability of powders would be different. Also, it could result into separation and segregation of the powders in the feeder. Thus, probably more of ZrO2 powders would be injected during the deposition. Also, some amount of alumina powders could be lost in plasma plume due to melting and vaporization of powders, as the Al2O3 melting temperatures are lower than compared to ZrO2 powders. The elemental composition of Al2O3 was aluminum and oxygen, with a low amount of impurities (Fe, Na) related to the nature of the employed feedstock powders, in addition to carbon due to absorption of the atmospheric gases. The weight percentage ratio of oxygen to aluminum (O/Al) slightly decreased from 1.00 to 0.97 with the enhancement of arc current for the alumina coatings.
The average surface roughness (Ra) and the root-mean-square roughness (Rq) of the Al2O3 coating deposited at 180 A was ~3.79 and 4.69 µm, respectively ( Figure 3). The surface roughness of the Al2O3 coating increased to Ra = 4.56 µm and Rq = 5.58 µm with the The used Al 2 O 3 and ZrO 2 feedstock powders were mixed mechanically. Thus, due to different size and shape of used Al 2 O 3 (non-regular shape) and ZrO 2 (mainly spherical shape) powders and their densities, the flowability of powders would be different. Also, it could result into separation and segregation of the powders in the feeder. Thus, probably more of ZrO 2 powders would be injected during the deposition. Also, some amount of alumina powders could be lost in plasma plume due to melting and vaporization of powders, as the Al 2 O 3 melting temperatures are lower than compared to ZrO 2 powders. The elemental composition of Al 2 O 3 was aluminum and oxygen, with a low amount of impurities (Fe, Na) related to the nature of the employed feedstock powders, in addition to carbon due to absorption of the atmospheric gases. The weight percentage ratio of oxygen to aluminum (O/Al) slightly decreased from 1.00 to 0.97 with the enhancement of arc current for the alumina coatings.
The average surface roughness (R a ) and the root-mean-square roughness (R q ) of the Al 2 O 3 coating deposited at 180 A was~3.79 and 4.69 µm, respectively ( Figure 3). The surface roughness of the Al 2 O 3 coating increased to R a = 4.56 µm and R q = 5.58 µm with the increase of arc current. As regards the average surface roughness of alumina-zirconia coatings, the R a increased from~2.28 to 3.43 µm, with increase in arc current from 180 to 220 A, respectively. The R q values had followed a similar trend with values at 180 and 220 A arc currents being 3.04 and 4.35 µm. Thus, with the increase of the plasma jet temperature, the roughness of the coatings increased, while the addition of ZrO 2 reduced the surface roughness. Usually, a higher melting degree of particle as plasma velocities increase results in a lower surface roughness of sprayed coatings [2,3,22]. The slightly higher surface roughness of the alumina-zirconia coating prepared at 220 A is due to the higher thickness value (as XRD (Figure 4) revealed only insignificant intensities of the steel substrate). The increase of roughness of Al 2 O 3 coating at higher arc current could be explained in such way: The Al 2 O 3 particles probably are only partly melted at 180 A and when it impacts the surface, partly melted particles split into smaller fragments and some of these fragments do not stick on the surface or are poorly adhered to it. As the temperature and velocity of the plasma are increased, the melting degree of the particles is enhanced, and a higher amount of particles would stick and be solidified on the surface. However, even at higher arc currents, the temperature required in order to fully melt the Al 2 O 3 particle was not reached. As a result, the surface roughness is slightly increased.
Coatings 2021, 11, x FOR PEER REVIEW 7 of 14 increase of arc current. As regards the average surface roughness of alumina-zirconia coatings, the Ra increased from ~2.28 to 3.43 µm, with increase in arc current from 180 to 220 A, respectively. The Rq values had followed a similar trend with values at 180 and 220 A arc currents being 3.04 and 4.35 µm. Thus, with the increase of the plasma jet temperature, the roughness of the coatings increased, while the addition of ZrO2 reduced the surface roughness. Usually, a higher melting degree of particle as plasma velocities increase results in a lower surface roughness of sprayed coatings [2,3,22]. The slightly higher surface roughness of the alumina-zirconia coating prepared at 220 A is due to the higher thickness value (as XRD (Figure 4) revealed only insignificant intensities of the steel substrate). The increase of roughness of Al2O3 coating at higher arc current could be explained in such way: The Al2O3 particles probably are only partly melted at 180 A and when it impacts the surface, partly melted particles split into smaller fragments and some of these fragments do not stick on the surface or are poorly adhered to it. As the temperature and velocity of the plasma are increased, the melting degree of the particles is enhanced, and a higher amount of particles would stick and be solidified on the surface. However, even at higher arc currents, the temperature required in order to fully melt the Al2O3 particle was not reached. As a result, the surface roughness is slightly increased. The XRD patterns of the as-sprayed Al2O3 and Al2O3-ZrO2 coatings are given in the Figure 4. The Al2O3 coatings consisted of hexagonal β-Al2O3, rhombohedral α-Al2O3 and cubic γ-Al2O3 phases. It was obtained that the beta-Al2O3 is the dominant phase. The peaks obtained at ~7.8°, ~15.7° and ~33.2° are assigned to sodium aluminum oxide (card No. 32-1033) with the β-Al2O3 (002), (004) and (107) orientations, respectively [12]. The diffraction peaks found at ~25.7°, ~35.2°, ~37.8°, ~43.4°, ~52.6° and ~57.6° are related to α-Al2O3 phase of (012), (104), (110), (113), (024) and (116) orientations, respectively [3,[12][13][14][15]. It should be noted that the initial alumina powders consisted of α-Al2O3 phase and sodium aluminum oxide (NaAl11O17) (beta phase), and the intensities of α-Al2O3 peaks were dominant [27]. Meanwhile, the low intensity peaks at 45.9° and 66.9° are related to existence of γ-alumina phases in the coatings ( Figure 4) [3,15]. It should be noted that the peaks at 43.7°, 50.7° and 74.6° are attributed to the steel substrate [12]. The increase in arc current slightly changed the intensities of the alumina phase peaks. The relative intensities of the dominant peaks were taken, and different phase ratios were calculated. It was obtained that the β-Al2O3/γ-Al2O3 ratio was slightly reduced from 6.0 to 5.9 with the increase in arc current. The α-Al2O3/γ-Al2O3 ratio decreased from 2.0 to 1.6, while β-Al2O3/α-Al2O3 ratio increased from 3.0 to 3.6 with the increase in arc current. It should be noted that the initial alumina powders consisted of α-Al 2 O 3 phase and sodium aluminum oxide (NaAl 11 O 17 ) (beta phase), and the intensities of α-Al 2 O 3 peaks were dominant [27]. Meanwhile, the low intensity peaks at 45.9 • and 66.9 • are related to existence of γ-alumina phases in the coatings (Figure 4) [3,15]. It should be noted that the peaks at 43.7 • , 50.7 • and 74.6 • are attributed to the steel substrate [12]. The increase in arc current slightly changed the intensities of the alumina phase peaks. The relative intensities of the dominant peaks were taken, and different phase ratios were calculated. It was obtained that the β-Al 2 O 3 /γ-Al 2 O 3 ratio was slightly reduced from 6.0 to 5.9 with the increase in arc current. The α-Al 2 O 3 /γ-Al 2 O 3 ratio decreased from 2.0 to 1.6, while β-Al 2 O 3 /α-Al 2 O 3 ratio increased from 3.0 to 3.6 with the increase in arc current. 220 A, respectively. While the t-ZrO2/m-ZrO2 ratio was ~3.8 in the feedstock powders. The m-ZrO2 phase fraction in total ZrO2 content was calculated using following equation [28]: C = 0.82 I + I 0.82 I + I + I where Cm is the content of m-ZrO2 phase, I is the intensity of diffraction peak, subscript monoclinic (m) and tetragonal (t) the phase type. The results indicated that the increase in arc current reduced the monoclinic phase content from ~17.6% to 13.2%. It should be noted that the m-ZrO2 phase fraction in the YSZ feedstock powders was ~27.1%. The reduction of the monoclinic phase in the coatings indicates the phase transformation during particle flight and solidification on the surface [19,26]. Also, the increase of the plasma jet temperature enhances the melting degrees of feedstock particles and leads to higher phase transformation values. With the increase in arc current from 180 to 220 A, the plasma temperature increases from 3400 to 3485 K and therefore, more Al2O3 feedstock particles are fully melted, or the fraction of the molten state would be higher. Thus, this resulted in the slight increase of the beta-Al2O3 and gamma-Al2O3 phase fraction in the Al2O3 coatings. It was demonstrated that with the increase of plasma arc current (plasma jet temperature) the content of the α-Al2O3 phase is reduced due to the higher degree of melting of the particles [1,3]. Tingaud   With the addition of zirconium oxide into the feedstock powders (Figure 4), the intensity of the alumina peaks in the XRD patterns of the Al 2 O 3 -ZrO 2 coatings was drastically reduced. The dominant peaks were attributed to the tetragonal (t-ZrO 2 ) and monoclinic phase (m-ZrO 2 ) of zirconia [14,15,20,28]. The XRD patterns of the coatings had not showed the cubic phase of zirconia. The peaks found at 2θ = 30.  [26,28]. The ratio of the highest peaks corresponding to t-ZrO 2 (~30.2 • ) and α-Al 2 O 3 (~35.2 • ) was hard to determine as α-Al 2 O 3 peak overlapped with t-ZrO 2 peak and was very weak. The increase in arc current also slightly enhanced the intensity of the γ-Al 2 O 3 peaks. The intensity ratio of t-ZrO 2 /m-ZrO 2 peaks were 6.7 and 9.5 for the coatings prepared at 180 and 220 A, respectively. While the t-ZrO 2 /m-ZrO 2 ratio was~3.8 in the feedstock powders. The m-ZrO 2 phase fraction in total ZrO 2 content was calculated using following equation [28]: where C m is the content of m-ZrO 2 phase, I is the intensity of diffraction peak, subscript monoclinic (m) and tetragonal (t) the phase type. The results indicated that the increase in arc current reduced the monoclinic phase content from~17.6% to 13.2%. It should be noted that the m-ZrO 2 phase fraction in the YSZ feedstock powders was~27.1%. The reduction of the monoclinic phase in the coatings indicates the phase transformation during particle flight and solidification on the surface [19,26]. Also, the increase of the plasma jet temperature enhances the melting degrees of feedstock particles and leads to higher phase transformation values. With the increase in arc current from 180 to 220 A, the plasma temperature increases from 3400 to 3485 K and therefore, more Al 2 O 3 feedstock particles are fully melted, or the fraction of the molten state would be higher. Thus, this resulted in the slight increase of the beta-Al 2 O 3 and gamma-Al 2 O 3 phase fraction in the Al 2 O 3 coatings. It was demonstrated that with the increase of plasma arc current (plasma jet temperature) the content of the α-Al 2 O 3 phase is reduced due to the higher degree of melting of the particles [1,3]. Tingaud et al. [22] found that the sprayed alumina and alumina-zirconia coatings mainly consisted of α-Al 2 O 3 phase and only a minor fraction of γ-Al 2 O 3 phase. The intensities of α-Al 2 O 3 peaks in our coatings is also higher compared to γ-Al 2 O 3 ( Figure 4). Usually, the γ-Al 2 O 3 phase dominates in the as-sprayed Al 2 O 3 or Al 2 O 3 composite coatings [1,2,14,15]. Several reasons could be attributed to explain such obtained results. Firstly, the cooling rate of molten or partly molten particles on the surface is insufficient. That is, the high heat flux delivered by the plasma flow to the surface of the coating keeps the particles in the liquid state for the longer time and delays the solidification of lamellae. As a result, the gamma phase can recrystallize back into the alpha phase [2,15,26,29]. The second factor is that a partial heat treatment takes place during the deposition of the coatings. The surface of the coating is locally heated by the plasma torch as it moves over the substrate. Thus, the locally heated regions composing of the metastable gamma phase can partially re-transform into the stable alpha phase [2,22]. The third reason is associated to the fact that the coating contains unmelted or partially melted particles identical to the initial feedstock, i.e., the alumina powder with the dominant alpha phase [22,26]. The final factor is that the partially melted alumina particles are represented by a liquid fringe (gamma phase) and a solid heart (alpha phase). The solid part acts like a germ and spreads into all the grains, which then have the alpha phase as resultant [22]. The surface morphology images of the coatings indicated that at higher arc currents, a more compact surface morphology with lower size particles was formed. So, a better contact between individual splats was formed and the cohesion would be higher [3,15]. It was demonstrated that the level of the micropore volume typically obtained at splat boundaries of the sprayed Al 2 O 3 coatings is reduced when the melting degree of the particles is enhanced [2,3].
The variation of the friction coefficient versus sliding time for the Al 2 O 3 and Al 2 O 3 -ZrO 2 coatings is presented in Figure 5. As the tribo-run progressed, the friction coefficients of all the coatings gradually increased with time, eventually reaching steady state values of~0.65-0.70 after 300-500 s. The further increase in sliding time resulted in only a marginal increase in the friction coefficients of the coatings and at higher time ranges (more than 1000 s), it fluctuated within a small range ( Figure 5). In the running-in stage, the friction coefficient is enhanced due to the increase in dry sliding contact area caused by the reduction in surface roughness of the coatings. When the high asperities are cut in the contact area, the friction coefficient become stable. Thus, it indicates the wear behavior of the coupled materials [2]. It should be noted that the friction coefficient curve versus time of the alumina coatings demonstrated slightly higher fluctuations compared to Al 2 O 3 -ZrO 2 . It was obtained that the larger surface roughness resulted in the higher fluctuation of the friction coefficient curve [1].
Coatings 2021, 11, x FOR PEER REVIEW 9 of 14 that the sprayed alumina and alumina-zirconia coatings mainly consisted of α-Al2O3 phase and only a minor fraction of γ-Al2O3 phase. The intensities of α-Al2O3 peaks in our coatings is also higher compared to γ-Al2O3 ( Figure 4). Usually, the γ-Al2O3 phase dominates in the as-sprayed Al2O3 or Al2O3 composite coatings [1,2,14,15]. Several reasons could be attributed to explain such obtained results. Firstly, the cooling rate of molten or partly molten particles on the surface is insufficient. That is, the high heat flux delivered by the plasma flow to the surface of the coating keeps the particles in the liquid state for the longer time and delays the solidification of lamellae. As a result, the gamma phase can recrystallize back into the alpha phase [2,15,26,29]. The second factor is that a partial heat treatment takes place during the deposition of the coatings. The surface of the coating is locally heated by the plasma torch as it moves over the substrate. Thus, the locally heated regions composing of the metastable gamma phase can partially re-transform into the stable alpha phase [2,22]. The third reason is associated to the fact that the coating contains unmelted or partially melted particles identical to the initial feedstock, i.e., the alumina powder with the dominant alpha phase [22,26]. The final factor is that the partially melted alumina particles are represented by a liquid fringe (gamma phase) and a solid heart (alpha phase). The solid part acts like a germ and spreads into all the grains, which then have the alpha phase as resultant [22]. The surface morphology images of the coatings indicated that at higher arc currents, a more compact surface morphology with lower size particles was formed. So, a better contact between individual splats was formed and the cohesion would be higher [3,15]. It was demonstrated that the level of the micropore volume typically obtained at splat boundaries of the sprayed Al2O3 coatings is reduced when the melting degree of the particles is enhanced [2,3]. The variation of the friction coefficient versus sliding time for the Al2O3 and Al2O3-ZrO2 coatings is presented in Figure 5. As the tribo-run progressed, the friction coefficients of all the coatings gradually increased with time, eventually reaching steady state values of ~0.65-0.70 after 300-500 s. The further increase in sliding time resulted in only a marginal increase in the friction coefficients of the coatings and at higher time ranges (more than 1000 s), it fluctuated within a small range ( Figure 5). In the running-in stage, the friction coefficient is enhanced due to the increase in dry sliding contact area caused by the reduction in surface roughness of the coatings. When the high asperities are cut in the contact area, the friction coefficient become stable. Thus, it indicates the wear behavior of the coupled materials [2]. It should be noted that the friction coefficient curve versus time of the alumina coatings demonstrated slightly higher fluctuations compared to Al2O3-ZrO2. It was obtained that the larger surface roughness resulted in the higher fluctuation of the friction coefficient curve [1]. The coefficient of friction (CoF) was calculated when a steady state was reached and the last 10% of CoF values with the corresponding standard deviations were considered. The average steady state coefficients of friction of the sprayed coatings are given in Figure 6a. It should be noted that the average CoF values of the Al 2 O 3 coatings produced at 180 and 220 A were 0.746 ± 0.010 and 0.723 ± 0.011, respectively. The addition of the ZrO 2 into the Al 2 O 3 feedstock powders only slightly changed the CoF values (Figure 6a). The alumina-zirconia coating prepared at 180 A demonstrated a CoF of~0.735 ± 0.015. With the increase in arc current, the average CoF value remained similar: 0.736 ± 0.042. The coefficient of friction (CoF) was calculated when a steady state was reached and the last 10% of CoF values with the corresponding standard deviations were considered. The average steady state coefficients of friction of the sprayed coatings are given in Figure  6a. It should be noted that the average CoF values of the Al2O3 coatings produced at 180 and 220 A were 0.746 ± 0.010 and 0.723 ± 0.011, respectively. The addition of the ZrO2 into the Al2O3 feedstock powders only slightly changed the CoF values (Figure 6a). The alumina-zirconia coating prepared at 180 A demonstrated a CoF of ~0.735 ± 0.015. With the increase in arc current, the average CoF value remained similar: 0.736 ± 0.042. The average wear rates of the alumina coatings deposited at 180 and 220 A were 4.55 × 10 −5 and 3.88 × 10 −5 mm 3 /Nm, respectively (Figure 6b). The addition of ZrO2 reduced the specific wear rate down to 1.25 × 10 −5 mm 3 /Nm for Al2O3-ZrO2 deposited at 180 A. The normalized wear rate was found to immeasurable, reasons owing to mere bulk-material transfer leading to plastic deformation, when the alumina-zirconia coating was deposited at 220 A. It should be noted that the wear rate of AISI 304 L steel was 1.17 × 10 −4 mm 3 /Nm.
The tribological tests indicated that the friction coefficient and specific wear rate of Al2O3 were slightly improved with the increase in arc current. Thus, the alumina particles would have reached higher temperatures in the plasma jet and when the particles impacted the substrate, a better contact between splats would have been obtained and the number of pores at the splat boundaries would have been reduced [3]. The Al2O3-ZrO2 coatings had lower specific wear rates than Al2O3-coatings under the same formation conditions. These results indicate that the addition of zirconia improved the interfacial adhesion between individual splats and the mechanical properties, as a result, the wear resistance was enhanced [15]. Liang et al. [9] observed that the presence of more pores will cause a high stress concentration and lead to the formation of cracks, resulting in a low wear resistance.
As the plasma temperature increased, the deposited coatings would be denser and less porous, due to a higher particle melting degree and higher particle velocities and would thus, demonstrate better tribological properties [2,22,26]. Such a trend is related to the reduction of the poorly adhered particles embedded into the coating matrix at higher arc currents. As a result, the amount of the stacking defect density is reduced and higher cohesion values between lamellae is reached, thus the friction behavior of the coatings is improved. Meanwhile, the addition of zirconia led to the production of even denser, cohesive, more homogenous and smoother surface coatings. The specific wear rate of alumina-zirconia coatings is lower, despite the slightly higher or similar friction coefficient values compared to the alumina coatings. Thus, the addition of zirconia into the alumina matrix improved the wear resistance of the coatings. It should be noted that the friction coefficient increases as the contact area is higher [2,22]. The roughness of the aluminazirconia coatings was ~25% to 40% lower when compared to the alumina coatings. Thus, the contact area between the coating and counterbody part is larger, resulting in the The average wear rates of the alumina coatings deposited at 180 and 220 A were 4.55 × 10 −5 and 3.88 × 10 −5 mm 3 /Nm, respectively (Figure 6b). The addition of ZrO 2 reduced the specific wear rate down to 1.25 × 10 −5 mm 3 /Nm for Al 2 O 3 -ZrO 2 deposited at 180 A. The normalized wear rate was found to immeasurable, reasons owing to mere bulk-material transfer leading to plastic deformation, when the alumina-zirconia coating was deposited at 220 A. It should be noted that the wear rate of AISI 304 L steel was 1.17 × 10 −4 mm 3 /Nm.
The tribological tests indicated that the friction coefficient and specific wear rate of Al 2 O 3 were slightly improved with the increase in arc current. Thus, the alumina particles would have reached higher temperatures in the plasma jet and when the particles impacted the substrate, a better contact between splats would have been obtained and the number of pores at the splat boundaries would have been reduced [3]. The Al 2 O 3 -ZrO 2 coatings had lower specific wear rates than Al 2 O 3 -coatings under the same formation conditions. These results indicate that the addition of zirconia improved the interfacial adhesion between individual splats and the mechanical properties, as a result, the wear resistance was enhanced [15]. Liang et al. [9] observed that the presence of more pores will cause a high stress concentration and lead to the formation of cracks, resulting in a low wear resistance.
As the plasma temperature increased, the deposited coatings would be denser and less porous, due to a higher particle melting degree and higher particle velocities and would thus, demonstrate better tribological properties [2,22,26]. Such a trend is related to the reduction of the poorly adhered particles embedded into the coating matrix at higher arc currents. As a result, the amount of the stacking defect density is reduced and higher cohesion values between lamellae is reached, thus the friction behavior of the coatings is improved. Meanwhile, the addition of zirconia led to the production of even denser, cohesive, more homogenous and smoother surface coatings. The specific wear rate of alumina-zirconia coatings is lower, despite the slightly higher or similar friction coefficient values compared to the alumina coatings. Thus, the addition of zirconia into the alumina matrix improved the wear resistance of the coatings. It should be noted that the friction coefficient increases as the contact area is higher [2,22]. The roughness of the alumina-zirconia coatings was~25% to 40% lower when compared to the alumina coatings. Thus, the contact area between the coating and counterbody part is larger, resulting in the slightly higher friction coefficient values. Tingaud et al. [22] also demonstrated that the addition of zirconia enhanced the friction coefficient of the coatings up to 5%, but reduced the wear rate up to nine times, when compared to alumina. The authors' observation of improvement in the tribological properties is attributed to the higher cohesive values and a denser structure of the sprayed coatings [15,22].
The SEM images of the wear scar morphology of the coatings is presented in Figure 7. The worn surface tracks of the alumina coatings were non-homogenous. SEM images indicated that the width of tribological tracks were~700-650 and~600 µm for the Al 2 O 3 coatings deposited at 180 and 220 A, respectively (Figure 7a,b). The surface of the coatings does not show formation of the micro-cracks or delamination of the splats. The wear track of the Al 2 O 3 -ZrO 2 coating was~500 µm wide and was quite homogeneous in all length when 180 A was used (Figure 7c). There were some microcracks and wear grooves formed and peeled-off particles (wear debris) on the surface (Figure 7e) The wear track became slightly narrower, non-continuous and wear grooves are not pronounced for the coating deposited at 220 A (Figure 7d,f). SEM images demonstrated that the material was removed only from the top portion of the hills' region of the coating, and that the coating was insignificantly damaged after the tribological test. The wear debris found on the worn surface of all coatings indicate the abrasive wear mechanism [1].
Several authors indicated that the wear resistance of the Al 2 O 3 -ZrO 2 [26], Al 2 O 3 -ZrO 2 -CeO 2 [15] and Al 2 O 3 -ZrO 2 -TiO 2 [29] coatings was improved due to enhancement of α-Al 2 O 3 phase content and reduction of γ-Al 2 O 3 phase amount. The main reason for reduction of wear rate and in some cases, the friction coefficient of coatings, is that the α-Al 2 O 3 phase has a higher hardness and elastic modulus, and also, fewer defects than γ-Al 2 O 3 [15,20,26,29]. The phase transformation of tetragonal to monoclinic ZrO 2 phase was observed when comparing the XRD data of the feedstock powder and the Al 2 O 3 -ZrO 2 coatings. It was observed that a significant volume expansion of approximately 3-5 vol.% occurs when the tetragonal phase is transformed to monoclinic [30]. As a result, a higher amount of micro-cracks and stress values between the steel substrate and the ceramic coating is observed [15,30]. Zhao et al. [14] obtained that the wear rate of Al 2 O 3 -ZrO 2 coatings decreases (despite slightly higher friction coefficient values), due to the reduction of m-ZrO 2 phase and a better inter-splat bonding. The increase in the arc current from 180 to 220 A results in the higher plasma temperature and a better heat transfer to the feedstock particle. Thus, a higher inter-splat bonding between alumina and zirconia splats is obtained. Reduction of the intensities of α-Al 2 O 3 and m-ZrO 2 phase peaks at higher arc currents is also attributed to the enhancement in the melted fraction of particles. Thus, the coatings sprayed at higher arc currents demonstrate an improved adhesion between the individual lamellae, are less porous, contain smaller sized pores and are denser. The reduction of pore sizes and semi-molten regions will suppress the pathways for the propagation of cracks and delamination of grains during the wear tests. The propagation of microcracks gradually peeled off the particles from the surface and the pulled-out (plowed) particles were crushed at the interaction zone between the coating and counterpart. Thus, the formed abrasive grains and wear debris could act as lubricants and slightly reduce the wear rate. However, the appearance of fluctuations on the friction curves of the Al 2 O 3 coatings ( Figure 5), indicates that the particles act as abrasives and increase the wear rate of the coatings. Thus, the sprayed Al 2 O 3 coatings demonstrated slightly higher wear rates compared to the Al 2 O 3 -ZrO 2 coatings (Figure 6b). Mehar et al. [29] indicated that at lower loads (15 N) plastic grooving and chipping were the dominant mechanisms of coatings wear. Wang et al. [15] showed that the CoF and wear rate of alumina-zirconia coatings were 0.74 × 10 −3 and 0.86 × 10 −3 mm 3 /Nm using 30 N, respectively. Several authors have indicated that the lower wear rate is associated with a better splat bonding adhesion, higher hardness, higher gamma phase content and lower porosity of alumina and alumina-zirconia coatings [2,14,15,22,29]. The wear grooves could be seen on the worn surface areas, the surface is much less removed in the alumina-zirconia coatings deposited at 220 A (Figure 7d,f). This indicates that the lowest amount of material is removed due to the highest splat bonding interactions and the wear rate is reduced. coatings deposited at 180 and 220 A, respectively (Figures 7a,b). The surface of th ings does not show formation of the micro-cracks or delamination of the splats. The track of the Al2O3-ZrO2 coating was ~500 µm wide and was quite homogeneous length when 180 A was used (Figure 7c). There were some microcracks and wear gr formed and peeled-off particles (wear debris) on the surface (Figure 7e) The wear became slightly narrower, non-continuous and wear grooves are not pronounced f coating deposited at 220 A (Figures 7d,f). SEM images demonstrated that the materi removed only from the top portion of the hills' region of the coating, and that the c was insignificantly damaged after the tribological test. The wear debris found on the surface of all coatings indicate the abrasive wear mechanism [1].

Conclusions
Alumina and alumina-zirconia coatings were formed by atmospheric plasma spraying using an air-hydrogen plasma. The increase in arc current increased the surface roughness of the Al 2 O 3 and Al 2 O 3 -ZrO 2 coatings. However, with the addition of ZrO 2 , the average surface roughness of the coatings was reduced up to 40%. The fraction of oxygen was reduced and an enhancement of Zr concentration was obtained at higher arc currents. The Al 2 O 3 coatings consisted of β-Al 2 O 3 , α-Al 2 O 3 and γ-Al 2 O 3 phases, and with the increase of arc current the α-Al 2 O 3 phase content was slightly reduced. The dominant phases in the alumina-zirconia coatings were t-ZrO 2 and m-ZrO 2 . It was found that the increase in arc current from 180 to 220 A, reduced the monoclinic ZrO 2 phase content from~17.6% to 13.2% in the Al 2 O 3 -ZrO 2 coatings. The friction coefficient slightly decreased from 0.746 to 0.723, while the specific wear rate reduced from 4.55 × 10 −5 to 3.88 × 10 −5 mm 3 /(Nm) for the Al 2 O 3 coatings, with the increase in arc current. The addition of zirconia into the alumina feedstock powders had no effect on the friction coefficient values of the coatings. The wear rate of the Al 2 O 3 -ZrO 2 coating prepared at 180 A, was reduced~3.6 times compared to Al 2 O 3 . The Al 2 O 3 -ZrO 2 coating deposited at 220 A demonstrated the highest wear resistance property and only an insignificant peeling of the hilltops on the surface of the wear scar zones was observed.