Development of a wear coefficient equation for the A A7075-B4C composite – steel interface

In this research, an attempt was made to develop a wear equation for specific wear regimes that differs with temperature, sliding velocity, applied load, and sliding distance. The experimental runs were designed with the L25 Taguchi orthogonal array, and the uniform dispersion of reinforcement was confirmed using a scanning electron microscope. The presence of reinforcement hinders dislocation movement led to an augmentation in the composites’ hardness, while an elevation in temperature resulted in a decline in hardness due to the reduction of Pierls stresses. Owing to the formation of a Mechanical Mixed Layer (MML), the wear rate decreases with addition of volume fraction of B4C particles until 7.5%, beyond this MML break down and wear rate transit from mild to severe due to the direct metal contact. At 50 °C, the wear mode was abrasive and delamination; at 150 °C, it was abrasive plastic deformation; and at 250 °C, it was plastic flow of materials. Grooves, micro pits, micro cracks, ploughing and resolidified material were the distinct feature observed on the worn surface morphology. The modified wear equation was developed by incorporating reinforcement effect, specific wear regimes, temperature-dependent factors, and functional parameters.


Introduction
Wear behavior at material interfaces has garnered significant attention due to its direct implications on the reliability and durability of engineering components [1].Abrasive wear, described by the erosion of material through the interaction of hard particles between the surface.Cutting, Ploughing, plowing and fragmentation of the materials surface are the characterization of abrasive wear [2].Adhesive wear occurred from the formation and failure of micro-scale juncture, which causes localized stress concentrations and material adhesion.Smearing, adhesive junctions, lumps and micro indentation are the distinct feature observed on the adhesive worn surface [3].Plastic deformation wear, as characterized by flow and yielding, contributes to material displacement and surface modification.Grooving, surface flattening, flow patterns and micro cracking are the microstructural features of Plastic deformation wear [4].Delamination wear occurred by the peeling of the surface categorized by surface flaking, material fracture, crack networks and void formation [5].Stir casting, compo casting, in situ fabrication, powder metallurgy and thermal spraying are the distinct methods utilized for the manufacturing of composites of which stir casting is simplest method and suitable for mass production [6].
The stirring speed, melting temperature, stirrer, reinforcement preheating temperature and pouring method are the various casting parameters which decides the uniform distribution of composites [7].Addition of flux improves the wettability of the composites by coating over the reinforced particles [8].Heat treated reinforced particles dispersed more uniformly than untreated particles [9].Optimal melting temperature eliminates the formation of intermetallic compounds and bottom pouring enhances the dispersion [10].Particulates, whiskers, and fibres are typical forms of reinforcement, whereas the characteristics of composites are determined by the volume fraction, size and form of the reinforcements.Silicon carbide, Boron carbide, graphite, Tungsten carbide and aluminium oxide are the prominent particles utilized as reinforcement [11].
Kumar et al investigated the abrasive behavior of TiB 2 -reinforced Al-4Cu composite and the findings revealed that increase in wear rate corresponding to an escalation in load under a constant sliding velocity [12].Venkataraman et al [13] explored fracture formation in dry wear tests on AA7075 reinforced with SiC MMC.Mahesh [14] utilized sliding wear tests to evaluate the wear behavior of Al-MMCs, incorporating B 4 C, SiC, TiC, and TiB 2 particles.Tests, conducted at loads of 80 N and 160 N using a Pin-on-Disk setup, demonstrated a lower wear rate for these Al-MMCs compared to pure Al.The study indicated minimal influence of reinforcement size and type on wear rate, while reinforcement content modestly affected wear resistance.Moreover, the addition of more SiC was observed to restrict the deformation of the Al 7075 alloy matrix against loading, further enhancing wear resistance.Abdelgnei et al [15] identified plastic deformation and delamination wear as the predominant wear mechanisms at elevated temperature.Enhanced wear resistance was observed through SEM analysis, revealing a stable layer comprising aluminum, oxygen, and iron on the alloy surface at 300 °C.Kumar et al [16] witnessed an enhanced wear performance in Al2219-B 4 C-graphite AMCs as the operating temperature rose from 50 to 150 °C, attributed to the development of an oxide layer on the sliding surface.Even though lot of work were performed on wear behavior of aluminium composites, development of wear equation with respect to the distinct wear regime was not performed.In this work an attempt was made to develop a novel wear equation with regards to the wear regime, temperature and wear functionalities.The invention of a unique wear equation in this paper addresses a critical research gap by giving a thorough knowledge of wear behaviour at material surfaces.By taking into account wear regimes, temperature effects, and wear functions, the equation provides a comprehensive method to characterising wear processes, which is critical for improving the reliability and durability of engineering components.

Experimental work
Materials AA7075 aluminum alloy has chemical composition depicted in table 1, has found its application in aerospace sector was selected as a matrix material.Its combination of high strength, lightweight properties, and corrosion resistance makes it a preferred choice for aircraft structural components, including fuselage panels, wing skins, and landing gear links.Boron carbide (B 4 C) with high hardness of 45 Gpa, low density of 2.51 g cm −3 possess excellent wear resistance make it an ideal choice for enhancing the mechanical properties of composites, particularly in applications requiring high wear resistance and hardness was selected as reinforcement.

Stir casting
Matrix material weighing 1 kg, was placed in a graphite crucible and heated to a temperature of 850 °C utilizing a stir casting furnace as shown in figure 1. B 4 C (2.5% volume fraction) had been heated to 250 °C to remove moisture before adding to the melt.The mixture was swirled with four arm stirrers at 1500 rpm for 120 seconds as shown in figure 2. At the liquidus temperature of 635 °C, AA7075 alloy encounters wettability issues with B 4 C.However, the addition of flux mitigates this problem, facilitating wetting at 850 °C.Hence, to improve wettability, an equivalent proportional volume fraction of K 2 TiF 6 flux was added to the melt, which was then stirred for 120 s.The potassium (K 2 ) component in K 2 TiF 6 aids in removing slag, while titanium (Ti) surrounds reinforcements to enhance wettability, the fluoride (F 6 ) component evaporates as fumes, owing to these multiple benefits it was preferred over other flux.The mixture was down poured in preheated mould made of die steel and its surface was machined to remove the surface defects.The same procedure was repeated to produce composites with volume faction of 5, 7.5% and 10% respectively.The casted samples were turned and faced to the required dimensions, as per the ASTM standards.

Wear test
The homogeneous dispersion of composites was confirmed through SEM.Rockwell hardness testing was performed according to the standard ASTM E18 [17] in which the applied load was100N and dwell time was 15 s.Pin on disc, wear test were performed on the samples as per ASTM G99 [18] standards by varying, temperature, load, reinforcement volume fraction, sliding velocity and distance, experimental runs were design using the Taguchi L25 orthogonal array as depicted in table 2. The wear experiments were conducted thrice on a steel counterface as shown in figure 3, and the wear rate was determined by calculating the average value.The analysis of the worn surface morphology was carried out using SEM.

Result and discussion
SEM confirmed that the reinforced particles were uniform distributed over the matrix material as portrayed in the figure 4. According to the Archard equation [19], volume of wear is directly proportional to the sliding distance and hardness of the contacting materials, as represented by the equation (1).
The experiments were conducted by varying the sliding distance (L), sliding temperature (T), and sliding velocity (V s ), by incorporating these parameters above equation can be modified as per equation (2).The function ( ) F V T , s is specific to the materials and operating conditions and it can vary for different materials and counterface conditions.
By incorporating the effect of reinforcement materials the wear equation can be rewritten as per equation (3), Let's denote the volume fraction of the reinforcing phase as V r and the hardness of the reinforcing phase as H r [20].
The wear rate reduces until the saddle point of 7.5 volume fraction, thereafter it increases as depicted in the figure 5.When sliding, these reinforced particles, abrades the material from both the counter face and composite pin results in the third body abrasion and formation of Mechanically Mixed Layer (MML).The experimental results were shown in table 3.These MML prevents direct contact between the two sliding body leads to the reduction of wear rate, confirmed through EDS analysis as shown in figure 6.Beyond the critical reinforcing percentage (7.5%), the wear rate increases attribute to the following factors (i) owing to agglomeration of particles delamination wear occurred [21] (ii) due to the reduction of hardness because of higher void percentage [22].Differentiating V with respect to the V , r The equation (4) is given by The negative sign indicates that an increase in the V r lead to a decrease in wear rate and vice versa.But in this case, Wear rate increases until 7.5% beyond that it reduces.The change in wear rate can be modified as per equation (5).

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The wear rate decreases initially with the addition of reinforcing particles until a certain critical volume fraction, and then increases again beyond a critical hardness.The composites became harder when reinforcement was present because it prevented dislocation movement; conversely, when temperature

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The wear rate increases linearly with raise in temperature and reaches the maximum of 0.03976 g min −1 , when slides at 150 °C [23].The worn surface morphology was examined to reveal about the methodology of wear in relation to change in temperature.When slides at 100 °C, the morphology showed deeper grooves, micro pits and resolidified materials as shown in the figure 7(a).Deeper grooves were formed because of the abrasive wear due to the shearing force and frictional interaction.The severity of wear depends on the depth of grooves (2-3 μm), influenced by wear process parameters and material properties.Micro pits were formed due to the sliding of abrasive particles, as the material is plowed away in the confined regions.The ploughed materials were redeposited over the surface results in the formation of resolidified material.At higher magnification of 1000x as depicted in the figure 7(b), morphology revealed materials were plastically flowed which confirmed that the composite pin attained its deformation temperature.The size of resolidified wear debris ranges from 3-5 μm and globules of size 0.5 μm were observed in the morphology.
The morphology of pin slides at 50 °C showed scratches, micro cracks and resolidified materials which confirmed that abrasive wear occurred as shown in figure 8(a).As the material undergoes sliding, localized stresses can exceed the material's fatigue strength, leading to the formation of microcracks.Third-body abrasion occurs when reinforced particles entrapped and interposed between the sliding surfaces.These particles act as an additional abrasive component and contributing to the formation of scratches on the material surface.The scratches oblige as tangible evidence of the abrasive action exerted by foreign particles, as they plow and gouge the material surface during the sliding process.At higher magnification cracks and deeper pits are clearly visible as shown in figure 8(b).Cracks formed as a result of the abrasive action and landed up in pits, which combined to peel the surface.Due to severe abrasive action, MML layer breakdown causes direct metal conduct, resulting in delamination wear as evident in surface morphology.
Composite pin when slides at 250 °C, worn surface morphology showed lumps, narrow grooves, plastic flow and eczema like surface as depicted in the figure 9(a).The formation of lumps was associated with the softening of the material under high-temperature conditions.As the material experiences elevated temperatures, it undergoes phase transformations, transitioning from a solid to deformation state.This transitional state can result in the agglomeration of material, forming lumps on the worn surface.When the softened areas cools, it creates irregular surface patterns reminiscent of the characteristic appearance of eczema.At higher magnification, resolidified material, groves of depth 1 μm were evident as shown in figure 9(b) which confirmed that the wear occurred through deformation and plastic flow of materials.By incorporating distinct mode of wear regimes and considering a temperature-dependent increase in wear rate equation can be modified as shown in equation (7).Here: k , p k , a k d are wear coefficients for plastic deformation, abrasive wear, and delamination wear, respectively.-( are functions representing the influence of sliding velocity and temperature on each wear regime.

( ) G T
is a temperature-dependent function representing the increase in wear rate with temperature.The wear rate initially decreases until a certain volume fraction (7.5%) due to plastic deformation and then increases beyond that due to abrasive and delamination wear, the wear equation modified accordingly as equation (8) [24]: Load, Reinforcement, and Sliding Distance function H(L, V , r D): ( ) ( )

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Differentiate the wear rate with respect to resistivity of temperature (ɑ) Abrasive Wear regime as equation (10)  To validate the developed equation, the wear equation was simplified to the form a Following with the derivative of simplified equation was computed as shown in equation (13).The predicted wear results were shown in the figure 10.  2. The wear equation incorporates the reinforcing phase's volume fraction and hardness, emphasizing their influence on wear resistance.The critical hardness was the determining factor, affecting the wear rate.Agglomeration of particles beyond the critical volume fraction and a reduction in hardness contribute to wear rate escalation.
3. The modified wear equation accounts for different wear regimes, including plastic deformation, abrasive wear, and delamination wear.It considers temperature-dependent functions for each regime, emphasizing the intricate relationship between temperature, sliding velocity, and wear rate.
4. Detailed analysis of worn surfaces at different temperatures reveals distinct morphological features.At 100 °C, deeper grooves and micro pits signify abrasive wear, while at 250 °C, lumps and eczema-like patterns indicate the material's softening and phase transitions.These morphological changes align with the wear equation predictions.

Figure 2 .
Figure 2. Line diagram of stir casting setup.

Figure 3 .
Figure 3. Line diagram of wear testing apparatus.

Figure 4 .
Figure 4. SEM image showed uniform dispersion of reinforced particles.

Figure 5 .
Figure 5. wear rate of the AA7075/B4C composites with change in functional parameters.

Table 2 .
Wear process parameters and its levels.

Table 3 .
Wear experimental runs and its results.
S.NoReinforcing percentage (%) Distance (m) Load (N) Velocity (m/s) Temperature (C) increased, Pierls stresses decreased results in reduction of hardness.The equation can further modify with respect to the effect of hardness as equation (6).Let's denote the critical hardness as H .crit