The material used in this study was a 42CrMo4 low-alloy steel with the following composition (wt%): 0.41% C; 0.77% Mn; 0.28% Si; 1.02% Cr; 0.16% Ni; 0.16% Mo; 0.25% Cu and balance Fe. The material was quenched at 850˚C, cooled in oil and tempered at 580˚C. The metallurgical structure obtained is a tempered martensite.

Cylindrical samples (15 mm × 16 mm) were cut out and rectified under identical conditions to obtain a uniform surface quality for all the studied parts. The average roughness (R_a) was 0.5 μm. One series of specimens were submitted to surface treatment by gas nitriding (temperature (θn) = 525˚C; Ammonia dissociation degree (τ) = 35%) under different time (12, 24, 36 and 48 h).

Nitrided layer were observed by using a scanning electron microscope (SEM) and X-ray diffraction (XRD). Investigations of compound layer were carried out using a Siemens Analytical D5000 diffractometer with Cu anode. The HV_0.05 micro hardness profile of the nitrided layers was obtained in the transverse section by means of Dhimadzu MMV-2 instrument. Three indentations were placed into the compound layer at equal depth in order to define the micro hardness profiles. Residual stress measurements were obtained using the X-ray diffraction method. The diffraction conditions are reported in Table 1.

The experimental set-up used to investigate wear significantly affects the mechanisms of wear recorded later on. The device used in this work is based on the initial by linear cylinder/plan contact represented in Figure 1; it is frequently seen in industrial applications such as cams, push rods and rollers. This configuration induces adhesive wear. The cylinder of diameter 200 mm is made from X200CrMoV12 steel. The parameters considered are:

• The normal loading (50 N < F_N < 225 N) and relative speed (3.1 m/s < V < 8.3 m/s) which are two energy parameters that influence the response of the material.

• The nature of the surface layer. Three distinct surface qualities were studied: untreated 42CrMo4 steel, treated steel with its compound layer, and treated steel without a compound layer. This layer was eliminated by grinding.

The weight loss was measured by weighing with a command precision of 10⁻⁴ g. All measurements were carried out under dry conditions. During each test the friction coefficient was recorded. Finally, the worn surface of nitrided samples was characterized by scanning electron microscope “SEM” observations to investigate the wear mechanisms.

Vickers micro hardness was evaluated at the cross-section of the specimens for

both treated and non-treated specimens. The results are plotted in Figure 2 against the distance from the surface; the depth hardness profile as a function of nitriding time. A maximum value of hardness of about 1100 HV was observed near the surface within the white layer. In the diffusion layer, the hardness decreases with the distance from the surface. For depths above 0.6 mm, the hardness remains at 355 HV. The micro hardness measurements were repeated for several specimens and no significant differences were found. Comparing the hardness of the treated and the untreated specimens for depths greater than 0.6 mm, similar values of about 355 HV were observed.

The metallurgical investigations carried out on the surface layer (Figure 3) and scanning electron microscopy “SEM” (Figure 4), show the existence of a compound layer, made up of two phases, γ' (Fe_3?4N) and ε (Fe₂₋N), and a hardened diffusion layer.

Table 2 shows the change of the compound layer thickness, case depth, sur-

*e_N: thickness of the nitrided layer. **e_C: thickness of the compound layer.

face hardness, and surface residual stress with nitriding time. The general trend for compound layer thickness is an increase of thickness with increasing time. The case depth increases with increasing time, as expected for diffusion-con- trolled growth. The surface hardness of 42CrMo4 steel was increased up by the gas nitriding process. The maximum surface hardness was observed at a treatment time of 24 h.

Results of adhesive wear tests on the 42CrMo4 steel, with and without gas nitriding treatments are show in Figure 5. Weight losses are less for the gas-

nitrided test specimens than for the quenched and tempered specimens which demonstrates the effectiveness of the gas-nitriding treatment on wear resistance. Wear resistance of the nitrided specimen was due to increased resistance against plastic deformation by increasing surface hardness. The improved wear resistance is considered to be related to the combined effect of the solid solution of Cr and the high chemical stable phases γ' and ε formed on the steel surface during gas nitriding.

When it is compared to the nitrided steels in Figure 6, it is observed that the 24 h nitrided samples resulted in improved adhesive wear resistance than this improvement is due to the higher surface hardness values. Nitrogen atoms take interstitial places among the parent α structure and form surface layers by expending the lattice. The possibility of CrN formation increases with increasing treatment time and therefore these layers dissolve. Chromium rich oxide film formed on the surface is a continuous layer and if its thickness, intensity and adhesion are well enough, it causes an increase at the wear resistance of the specimens. Figure 7 shows results of adhesive wear test with and without compound layer. The wear loss increases with the elimination of the compound layer. It can be explained to the effects of hardness on wear. This is confirmed for all loading levels applied and for all speeds used. It has been observed that wear rate varies as a function of the tests conditions due to the presence of different wear mechanisms. Thus, for short tests conditions wear rate depends on two mechanisms: plastic deformation and adhesive wear, whereas for large tests conditions the mechanisms controlling wear rate are abrasive and oxidative wear.

An analysis of the wear mechanisms of the nitrided parts was carried out using SEM of the wear debris and the wear traces on the nitrided samples. The SEM observations of the wear traces on the nitrided samples made it possible to analyze the wear mechanisms on nitrided surfaces.

Figure 8 shows SEM micrographs of low alloy 42CrMo4 steel typical worn surface. It shows wide, smooth and continuous wear grooves and flakes along the wear track, where a little metal oxide is observed. Moreover these observations highlight the existence of the phenomenon of chipping for the weak conditions of the wear tests with the appearance of micro cracks at the level of the white layer. Figure 9 shows a wear crack with a compound layer. In the initial period of sliding, the brittle compound layer with high stress fractured and then transformed into abrasive particles. Higher magnification reveals that this layer initially cracked and broke into pieces. The SEM analysis of the wear remains enabled us to note that the wear particles with compound layer are detached from scales because of the porosity and the brittleness of this layer.

These observations revealed that the compound layer is delaminated as dice during the first cycles of wear. This observation is confirmed for all the loading levels applied and for all speeds used, as illustrated in Figure 10. In this figure, one notes that the worn thickness is greater than the thickness of the compound layer even for the low loading levels. SEM images of the wear debris collected during the first cycles are presented in Figure 11. The general aspect observed shows that this debris was removed in the form of plates per delamination. It is noticed that for severe conditions the number of folds multiplies with the presence of micro cracks on the level of the compound layer. Indeed the matter re-

Figure 9. Wear debris composed of plastically deformed particles.

moved during the wear test is stuck on surface used creating the third body necessary to improve the wear resistance of the nitrided parts (Figure 12).

The SEM observations of the transition zone worn surfaces/no worn surfaces showed that the matter is removed in the shape of the plates by delamination. One can deduce that the compound layer has a weak adhesion with the sub-

Figure 11. Morphology of the wear fragments of the compound layer collected at the beginning of the test at F = 50 N; V = 3.14 m/s.

strate, which can be explained by the porosity of this layer, and also its very low ductility estimated at 1%. However, this low level of adhesion is only apparent when insistences of the shear stress are produced during wear (Figure 13). The SEM analysis concerning the wear morphology of the parts used under very severe conditions revealed the existence of the burns which increase according to the test condition (Figure 14). When a higher load level is applied, the dominating mechanism becomes worn by oxidation. When the compound layer was eliminated, two types of specific wear mechanisms were observed depending on the load level. The first type occurred at low load levels and corresponds to the phenomena of adhesion and abrasion as shown by SEM (Figure 15). In all cases and for the various loading levels, wear debris formed initially from the compound layer are trapped in the contact. These are composed of γ' and ε iron nitrides and constitute hard particles which contribute to further wear of the diffusion layer by abrasion. In these cases the wear debris take a nodular form (Figure 16), and the worn surface presents traces of hammering (Figure 17) caused by contact between the three bodies under conditions of flow that remain difficult to control.