Thin Films: Study of the Influence of the Micro-Abrasive Wear Modes on the Volume of Wear and Coefficient of Friction

The purpose of this work is to study the influence of the micro-abrasive wear modes on the behaviors of the volume of wear (V) and of the coefficient of friction (μ) of thin films submitted to micro-abrasive wear. Experiments were conducted with thin films of TiN, TiAlN, TiN/TiAlN, TiHfC, ZrN, and TiZrN, using a ball of AISI 52100 steel and abrasive slurries prepared with black silicon carbide (SiC) particles and glycerine. The results show that the abrasive slurry concentration affected the microabrasive wear modes (“grooving abrasion” or “rolling abrasion”) and, consequently, the magnitude of the volume of wear and of the coefficient of friction, as described: (i) a low value of abrasive slurry concentration generated “grooving abrasion,” which was related to a relatively low volume of wear and high coefficient of friction, and (ii) a high value of abrasive slurry concentration generated “rolling abrasion,” which was related to a relatively high volume of wear and low coefficient of friction.


Introduction
The micro-abrasive wear test by rotating ball ("ball-cratering wear test") is an important method adopted to study the micro-abrasive wear behavior of metallic, polymeric, and ceramic materials.Figure 1 presents a schematic diagram of the principle of this micro-abrasive wear test, in which a rotating ball is forced against the tested specimen in the presence of an abrasive slurry, generating, consequently, the called "wear craters" on the surface of the tested material.
Initially, the development of the ball-cratering wear test aimed to measure the thickness of thin films (Figure 2a and b) [1], which can be made using the equations detailed in Ref. [2].Because of the technical features, this type of microabrasive wear test has been applied to study the tribological behavior of different materials [3][4][5], for example, in the analysis of the volume of wear (V), coefficient of wear (k), and coefficient of friction (μ) of thin films [2,[6][7][8][9][10].
As a function of the abrasive slurry concentration, two micro-abrasive wear modes can be usually observed on the surface of the worn crater: "grooving abrasion" is observed when the abrasive particles slide on the surface, whereas "rolling abrasion" results from abrasive particles rolling on the specimen's surface.
Figure 3a [11,12] and Figure 3b presents, respectively, images of "grooving abrasion" and "rolling abrasion." Many works on coefficient of friction (μ) during abrasive wear and other types of tests are available in the literature [13][14][15][16][17][18][19], but only a few were dedicated to the coefficient of friction in ball-cratering wear tests [2-4, 10, 11].In particular, Shipway [20] has studied the coefficient of friction in terms of the shape and movement of the abrasive particles, Kusano and Hutchings [21] presented a theoretical model for coefficient of friction in micro-abrasive wear tests with "free-ball" equipment configuration, and Cozza et al. [2-4, 11, 22] measured the tangential force developed during tests conducted in a "fixed ball" equipment configuration, which allowed direct calculation of the friction coefficient by the ratio between the tangential and normal forces.Besides, using a proper electronic instrumentation, Cozza et al. [2,[23][24][25] have studied and measured the behavior of the coefficient of friction in thin films in ball-cratering wear tests; however, in those works [2,[23][24][25], the test sphere has reached the substrate.Analyzing and studying important researches regarding to tribological behavior of materials submitted to micro-abrasive wear test conditions [7][8][9]26], the purpose of this work is to report the influence of the micro-abrasive wear modes on the behaviors of the volume of wear (V) and coefficient of friction (μ) of thin films submitted to micro-abrasive wear tests by rotating ball.

Ball-cratering wear test equipment
A ball-cratering wear test equipment with free-ball mechanical configuration (Figure 4 [27]) was used for the micro-abrasive wear tests, which has two load cells: one load cell to control the "normal force" (N) and one load cell to measure the "tangential force" (T) that is developed during the experiments.The values of "N" and "T" are read by a readout system.The abrasive material was black silicon carbide (SiC) with an average particle size of 3 μm; Figure 5 [4] presents a micrograph of the abrasive particles (Figure 5a) and the particle size distribution (Figure 5b).The abrasive slurries were prepared with SiC and glycerine.

Methods
Table 1 presents the values of the test parameters defined for the micro-abrasive wear experiments.
The normal force value defined for the wear experiments was N = 0.4 N, combined with two abrasive slurries concentrations (C), C 1 = 5% SiC + 95% glycerine and C 2 = 50% SiC + 50% glycerine (volumetric values), with the purpose to produce, respectively, "grooving abrasion" and "rolling abrasion" on the surfaces of the thin films.The ball rotational speed was set to n = 70 rpm.
All tests were non-perforating, e.g., only the thin films were worn.The normal force (N) was constant during the tests; the tangential force (T) was monitored and registered during all experiments.
The volume of wear (V) and the coefficient of friction (μ) were then calculated using Eqs.(1) [1] and (2), respectively; "d" is the diameter of the wear crater, and "R" is the radius of the ball:

Results and discussion
Figures 6 and 7 show examples of worn surfaces obtained in the experiments; in all wear craters, the maximum depth (h) observed was, approximately, h ≈ 8 μm. Figure 6 displays the action of "grooving abrasion," characteristic of C 1 = 5% SiC + 95% glycerine; Figure 7 displays a wear crater under the action of "rolling abrasion," reported for the abrasive slurry concentration C 2 = 50% SiC + 50% glycerine.These results qualitatively agree with the conclusions obtained by Trezona et al. [28], in which low concentrations of abrasive slurries (<5% in volume of abrasive material, approximately) favor the occurrence of "grooving abrasion" and high concentrations of abrasive slurries (>20% in volume of abrasive material, approximately) favor the action of "rolling abrasion." The actions of the micro-abrasive wear modes showed an important influence on the volume of wear and on the coefficient of friction of the thin films studied in this research.A significant increase in the volume of abrasive particles from C 1 = 5% SiC + 95% glycerine to C 2 = 50% SiC + 50% glycerine (causing, consequently, the micro-abrasive wear transition from "grooving abrasion" to "rolling abrasion") caused an increase in the volume of wear and a decrease in the coefficient of friction.
Figures 8 and 9 show the behaviors of the volume of wear (V) and coefficient of friction (μ) as a function of the micro-abrasive wear modes; the maximum errors observed were V = 0.4 × 10 −3 mm 3 and μ = 0.1, for the volume of wear and coefficient of friction, respectively.
The values of the volume of wear reported under conditions of "rolling abrasion" (high-abrasive slurry concentration, C 2 = 50% SiC + 50% glycerine) were higher than the values of the volume of wear reported under conditions of "grooving

Figure 1 .
Figure 1.Micro-abrasive wear test by rotating ball: a representative figure showing the operating principle and the abrasive particles between the ball and the specimen; "h" is the depth of the wear crater.

Figure 2 .
Figure 2. Examples of wear craters generated on coated system: (a) multilayer and (b) thin film of TiN.

Figure 4 .Figure 5 .
Figure 4. Ball-cratering micro-abrasive wear test equipment used in this work: free-ball mechanical configuration, able to acquire, simultaneously, the "normal force N" and the "tangential force T."

Figure 6 .
Figure 6.Occurrence of "grooving abrasion" on the surface of the thin film of TiN.

Figure 7 .
Figure 7. Occurrence of "rolling abrasion" on the surface of the thin film of TiN.

Table 1 .
Test parameters selected for the ball-cratering wear experiments.