Zhe role of zirconia additions on the microstructure and corrosion behavior of Ni-Cr dental alloys

This study investigates the effect of zirconia addition with different percentages on Ni-Cr alloy using a powder metallurgy technique. The scanning electron microscope with energy dispersive spectroscopy is used to analyze the microstructure of Ni-Cr and Ni-Cr-x ZrO2 (where x is 3, 6, 9 wt.%) alloys. The x-ray diffraction method is used to determine the phases for Ni-Cr and Ni-Cr/ZrO2 alloys. The corrosion and ion release tests were also achieved using potentiodynamic polarization and atomic absorption spectroscopy. According to the microstructural investigation, the results found that the Ni-Cr alloy’s grain size tends to reduce with the addition of ZrO2 with obtaining the smallest grain size with 6 wt.% addition. The potentiodynamic polarization exhibited that the modified alloys are more resistant to corrosion in the saliva medium than the Ni-Cr alloy. Furthermore, the role of ZrO2 in dissolution test shows the development in the passive layer’s resistance compared to unmodified alloys with reducing the Ni release from 2.5 ppm to 0.2 ppm. It can conclude that the addition of ZrO2 has significantly improved Ni-Cr’s biocompatibility and extend the area of implementations.


Introduction
Metal alloys have been used in various dental applications for many decades. Metal alloys are still common in the dental practice, and implants, such as bridge and porcelain-fused-to-metal (PFM) crowns due to their favorable mechanical properties and reasonable price [1,2], despite being available in many non-metallic materials. However, chronic adverse reactions, for example, gingival pigments, gingival atrophy or hyperplasia, and urticaria have been reported with minor and significant allergic responses [3,4]. Various risk factors, including the potential for cytotoxicity, must also be considered when carrying out biological assessments. Previous research on industrial alloys shows that continuous corrosion is typically due to multiple individual variables [5]. As a result, metals could be released from dental alloys as ions into surrounding tissues and saliva and then likely diffused across the body through the blood, causing possible cytotoxic effects for extended periods on local tissue or distant organs [6]. The biological impact or trace metals produced in peripheral blood and distant organ by dental alloys were studied extensively.
Jakobsen et al reported that metals released from the Co-Cr-Mo alloy accumulated in rats after nine months in the liver and kidney [7]. After six months in rabbits with Ni-Cr crowns, Zhu et al observed elevated Ni and Cr in blood [8]. In addition, Mcginley and colleagues have shown that Ni can be the primary cause of cytotoxicity and apoptosis and that Ni's inhibitory effect is partly due to its ability to induce apoptosis via caspase-3 activation [9,10]. Recent research about nickel-chromium alloy's possible toxicity on the kidneys has attracted much attention in China. This results in patients requesting the removal of dental restorations with heightened fears regarding dental alloys' safety.
Moreover, patients who have had PFM dental implants often need to get their implants replaced due to complications such as framework fractures and veneer chipping (such as caries and abutment tooth fracture) [11,12]. The accumulation of trace metals and potentially harmful effects can be minimized or terminated by removing dental alloys needs examination. In dental applications, Ni-Cr alloys are used for long periods and Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. substituted by patients who cannot be removed [13]. While the concentration of trace metals in dental patients is not proven to have any adverse effects, trace metals can be toxic and potentially affect primary biological and cellular functions. Therefore, information about the amounts of trace metals in distant organs affected by Ni-Cr dental alloys greatly benefits researchers. Furthermore, the Ni-Cr alloys' corrosion behavior properties mainly depend on forming the passive layer [11]. Therefore, the performance of the Ni-Cr alloys corrosion behavior in the dental applications is vital in order to understand their bio-characteristics and their compatibility are represented by the benefit-risks of the health of these alloys.
On the other hand, structural ceramics are widely used in medical implant applications as an efficient replacement for metallic dental prostheses' infrastructure. Zirconia is one of the best ceramics that emerged as a promising and multipurpose material to be used in several medical applications because of its excellent optical, mechanical, and biological properties [14,15]. These properties have accelerated their path for CAD/CAM technology usage for several prosthetic treatment types [16]. Zirconia is defined as a biomaterial that showed a promising future due to its high fracture toughness and mechanical strength and the continuous advantages over other types of materials due to the mechanisms of the toughness transformation in their microstructural change [17].
However, the addition of ZrO 2 as the third element to the Ni-Cr alloy has yet been studied. Thus, this work investigates the effect of ZrO 2 addition, for three weight percentages, to Ni-Cr alloy. The ion releasing and corrosion behavior in artificial saliva are analyzed in order to discover the capacity of using the presented alloy as restorative materials.

Materials used
The materials used to prepare and reinforce the Ni-Cr alloys in this work were demonstrated in table 1 with average particle size, purity, and supplied company. The particle size was measured using a better size 2000 laser particle size analyzer and a handheld (XRF) analyzer type (DS-2000), USA, to determine powders' purity. The sample code and the detailed description of the prepared samples used in the present study are shown in table 2.

Powder mixing
The powders are mixed using the electrical rolling mixer type (STGQM-1/5-2). To ensure the mixing of powders, steel balls are used in different sizes with minimal alcohol added to eliminate the friction and oxidation generated during the mixing process for 6 h.

Powder compacting
In this stage, an electrical hydraulic press is used for compacting the blended powder to get green samples of 13 mm and 5 mm in diameter and thickness, respectively, to be used in the test. To reduce the friction resulting from the direct contact between the walls of the die and green compact, graphite powder was used as a lubricant. Graphite applications are also used to avoid the initiated cracks resulted from green compact ejection. The green   show the sintering process diagram and the final shape of sintered samples. All samples were ground after sintering process utilizing grit silicon carbide papers of (180, 220, 320, 600, 800, 1000, 1200, 1500, 2000, and 2500), then they were polished utilizing a diamond past of 15 μm and metallographic polishing pads to obtain a bright mirror finishing finally. Etching at room temperature was conducted. Etching solution having a chemical composition listed in table 3 was used [18]. The samples were cleaned with water and dried after the etching process.
The phase analysis was performed using x-ray diffraction (XRD6000) with CuKa radiation at 40 kV and the scanning step of 5°per minute. The surface morphology and chemical analysis were characterized using (SEM, VEGA3 LM) equipped with INCA-AE350 Energy-dispersive x-ray spectroscopy (EDX).
The modified and unmodified alloy's corrosion characteristics were measured with Tafel test in an artificial saliva solution at 37 ℃ and 6.7 pH. The chemical composition of artificial saliva is illustrated in table 4 [19].
According to the ASTM standard, the Pt, SCE, and the sample were used as a counter, reference, and working electrodes, respectively. The potentiodynamic polarization curves were constructed. Using Tafel plots, the corrosion potential and corrosion current density (I corr ) were estimated by anodic and cathodic branches. The test was carried out with potential being stepped at scanning rate equals 0.4 mV s −1 from an initial potential that equals 250 mV below the potential of an open circuit. The scanning was continued up to 250 mV above the potentials of the open circuit. The measured rate of corrosion can be obtained as following [20,21]: Where Ew, ρ, Icorr, mpy refer to the equivalent weight (g/eq.), density (g cm −3 ), current density (μA cm −2 ), and (mpy) (mils per year), respectively. The constant 0.13 represents the metric and time conversion factor. According to [22], the Ni and Cr ions content leached into saliva from the samples was determined using dissolution tests to measure ions in sample solutions submerged at 37 ℃ for five weeks. A sample of each base alloy and modified with ZrO 2 samples were immersed in 50 ml of saliva in bottles made from polypropylene (PP). These bottles were closed firmly and incubated in the thermostatic chambers for five weeks at 37°C. Every three days, the bottles were shaken quietly for a couple of seconds. After five weeks, the bottles' saliva and solution were analyzed using atomic absorption spectroscopy to determine the amount of Ni and Cr ions leached from each sample, according to Joan et al [23].   Figure 2 displays the SEM micrographs of the sintered modified and unmodified Ni-Cr alloys at 950 ℃ for seven hours. The results revealed that the sintered samples' microstructure of Ni-Cr-6 wt.% ZrO 2 and Ni-Cr-9 wt.% ZrO 2 exhibited a structure with two phases; Ni 3 Mo and ZrO 2 that formed in the matrix structure of δ (Cr, Ni). It was also clear that the sintered samples' grain size was obtained a smaller grain size with 6 wt.% of ZrO 2 ; further addition of ZrO 2 may increase the grain size. The alloying element of Ni, Cr, and Zr percentage addition was distributed uniformly in Ni-Cr's microstructure, as shown in the elemental mapping in figures 2(d) and (e).

Microstructural observation
According to the EDX results (figures 2(f) and (g)), the percentage of the alloying element like Zr was varied according to the additional amount with identifying the percentage of oxygen, which shows an increase in the volume oxygen as the Zr addition increased from 6 wt.% to 9 wt.%. This is in good agreement with the elemental mapping in figures 2(d) and (e). The XRD scanning profile of the Ni-Cr-6 wt.% ZrO 2 and Ni-Cr-9 wt.% ZrO 2 alloys after been sintered at 950°C for 7 h are shown in figures 3(a) and (b). The presentence peaks were reflected in the alloying formation rather than elements, in which it is proven that the 7 h sintering was sufficient enough   to complete the transformation. On the other hand, forming the alloying compounds rather than individual elements reduces the toxicity effect into the body as a future biomaterial's implementations [24]. Furthermore, the observed Ni 3 Mo and δ(Ni-Cr) phases' peak intensity tends to increase as ZrO 2 addition increased.

Porosity and density after sintering
The porosity and density of Base alloy Ni-Cr and Ni-Cr-3, 6, and 9 wt. % ZrO 2 after sintering have been determined according to the ASTM B-328 standard [25]. Final porosity is the measuring for closed and open pores in the volume sintered samples. The porosity of the base alloy is 16.21 % and this represents the open and closed pores in the sample. It was noticed that ZrO 2 addition decrease the porosity value to 13.46, 11.21, and 10.96% with 3, 6, and 9 wt. % ZrO 2 addition respectively due to the small particle size of zirconia that can move and full the spaces between the settled particles during compaction which gives lower porosity after the sintering process. The presence of pores decreases the strength because the pores act as stress concentration regions and crack initiation regions also affect the corrosion resistance of the alloy because the pores may extend to the internal surface and increase the corrosion occur due to increasing the surface area [26,27]. The existence of ZrO 2 addition leads to an increase in the contact areas between the particles, however, this will increase the diffusion between the particles in the sintering process which leads to a decrease in the porosity and increase the density. It was observed that the density of sintered sample for the Base alloy Ni-Cr is 8. 012 g cm −3 and for Ni-Cr-3, 6, and 9 wt. % ZrO 2 are 8.246, 8.692, and 8.842 g cm −3 . Figure 4 presents the potentiodynamic polarization curves of Ni-Cr alloy without and with ZrO 2 addition of in synthetic saliva solution at 37 ℃. The corrosion parameters, such as corrosion potential, corrosion current, polarization constant (β c , β a ), and corrosion current densities, were extracted from these curves and tabulated in table 5. It can be observed that there is a significant shift toward lower current densities of the polarization curves for the sample with different additives of ZrO 2 . The Icorr for the Ni-Cr-3 wt.% ZrO 2 is around 0.20 μA cm −2 , while, Icorr for Ni-Cr-6 wt% ZrO 2 and Ni-Cr-9 wt% ZrO2 is around 0.03 μA cm −2 and 0.09 μA cm −2 , respectively, which are much lower than Icorr for the bare alloy, which around 4.51 μA cm −2 .

Potentiodynamic polarization
These results indicate the stability of the Cr 2 O 3 passive layer against dissolution, and these findings are in agreement with another research in [28]. The corrosion current and the calculated corrosion rate are relative measurements of corrosion, and they refer to the material loss magnitude throughout the corrosion process. Therefore, the higher current density and the calculated corrosion rate cause more materials to lose [28]. In Ni-Cr alloys, Cr plays a crucial role in protecting the surface via forming a Cr 2 O 3 layer that leads to passive these alloys. However, corrosive ions in saliva can destroy such passive layers. On the other hand, the results also revealed that the Ni-Cr with 3, 6, and 9 wt.% ZrO 2 alloys are achieved a lower corrosion rate compared with the Ni-Cr alloy. This enhancement may attribute to the addition of ZrO 2 to the base alloy leads to corrosion potential (E corr ) shifting towards the active direction with a reduction in the density of the corrosion current, specifically in the existence of 6 and 9 wt% ZrO 2

Ni and Cr ions release measurement
Ni's ion's release may further stimulate nickel sensitization or provoke allergic contact dermatitis [29]. The Ni ions test release is applied to ensure that Ni-Cr alloys can be used in human bodies. Figure 5 illustrate Ni and Cr ions' concentration for the Ni-Cr alloys with and without ZrO 2 additions after five weeks of immersion in synthetic saliva at a temperature of 37 ℃. Measurable released ions of Ni from the based alloy of Ni-Cr is about 2.5 ppm, and this amount led to decreased as the ZrO 2 was added with obtaining the minimum value of 0.3 ppm with 6 wt.% of ZrO 2 . This improvement may be attributed to the ability of ZrO 2 to be incorporated with Ni in the matrix to reduce the metal dissolution and then permits for Cr 2 O 3 to be continued.
However, the Cr ion releases, shown in figure 5, indicate the highest percentage of Cr ion release with Ni-Cr's unmodified alloy. The release amount reduced as the ZrO 2 addition increase with an optimum value of 0.2 ppm with 6 wt.% addition, and as the ZrO 2 increased to 9 wt.%, the ion release of Cr tends to increase as well. On the