Issue 78

N. Mohan et alii, Frattura ed Integrità Strutturale, 78 (2026) 129-145; DOI: 10.3221/IGF-ESIS.78.08

reported that at 6h of boriding time the wear rates varied in the range of 9.4 to 4.8 × 10 -6 mm 3 /Nm against WC-Co counterbody at an applied load of 10N [17]. L.A. Arteaga-Hernandez et al. studied the tribological behavior of borided AISI 316 stainless steel at varied boriding temperatures (850, 950 and 1050°C) and durations (2, 4 and 6 h). They reported that at 950°C and 6 h duration of boronizing, the wear rates were in the range of 7.87 × 10 -6 mm 3 /Nm which increased to 4.02 × 10 -5 mm 3 /Nm at 1050°C. The sliding wear tests were carried out against an alumina counter-body at an applied load of 20N [18]. Similar trend of decrease in wear rate with increase in boronizing temperature is seen in the present study.

Figure 7: Wear rates of (a) samples and (b) counterbody, bare: (i) 13Cr4Ni MSS and (ii)16Cr-5Ni MSS and boronized 13Cr4Ni and 16Cr5Ni MSS boronized at (iii - iv) 1000 ℃ and (v – vi) 950 ℃ , respectively, in dry sliding wear condition. The SEM micrographs seen in Fig.8 demonstrate the wear mechanism of the boronized samples in the dry sliding wear condition. A clear distinction in the wear mechanism of samples boronized at 950 and 1000°C is evident. The temperature of boronizing significantly influences the wear mechanism. For both grades of MSS boronized at 950°C, severe deformation of the worn surfaces is seen along with few micro-cracks and pits while at 1000°C severe fracture is the dominant wear mechanism along with the presence of few micro-cracks. Both grades of MSS when boronized have hardness between 1680-1805 Hv (Tab. 2.) which is comparable to the value of hardness of the Al 2 O 3 counter-body (1700 2000 Hv). As both the counter-body and the boronized MSS have similar hardness, it is difficult for the counter-body to sink into the surface of the sample and limits the material removal of the substrate surface due to repeated sliding. For MSS boronized at 950°C, the counter body doesn’t break the microscopic asperity points, rather, these points tend to adhere and when the counter-body slides repeatedly, the shear forces break these asperity junctions (plastic deformation) and may transfer the material onto one of the surfaces (spalling). Also, the broken fragment if not transferred onto the material will act as new surface between the sliding counterbody and the substrate and cause third body wear contributing to enhanced wear of material. In this case, the SEM micrograph of the counterbody (figs. 9 (a-b)) clearly shows material transfer in both grades of MSS boronized at 950°C and the micrographs of the substrate show severe surface fatigue and spalling due to which material suffers plastic deformation (fig. 8 (c and f)). The increase in surface roughness due to plastic deformation and third body wear due to debri particles could also explain the higher COF and wear rate of these samples. Further, in the case of MSS boronized at 1000°C the mechanism of wear is combination of several factors. The hardness is high, fracture toughness is low, and there is presence of additional boride phases in boronized MSS at 1000°C in comparison with their counterparts at 950°C. Higher hardness resists wear, however, the low fracture toughness allows for development of cracks and easy breakage of the material on the surface. Also, from the previous literature it is noticed that presence of additional borides contributes for an enhancement in the residual stress values. The counterbody slides over the substrate surface and after repeated sliding breaks the asperity points. The broken wear debris now contributes to three body wear and abrades the surfaces. However, instead of adhesion at the contact of asperity point, detachment of a thin layer of material is observed due to the inherent residual stress and low fracture toughness of the material. The SEM micrographs of the counterbody (figs. 9 (c-d)) show significant abrasive marks on their surface. The non-circular shape of the wear scar on the counterbody indicates that the accumulation of loose wear debri particles causes an even contact between the sliding surfaces and as these particles are hard, they cause more abrasion of the surface. From the SEM

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