Thermal stability of white layers intended as process-induced functional coatings
Paper i proceeding, 2020
Hard turning and grinding are finishing processes for the manufacture of precision mechanical components. However, a major concern with respect to service performance (e.g. in bearing races receiving high contact stresses) is white layer (WL) formation on the component surface. WLs are microstructure alterations which are generally considered detrimental to fatigue life and wear resistance. But WLs can also be regarded as process-induced functional surfaces which result in improved wear resistance and produce compressive residual stresses which may significantly increase the component’s fatigue life. In fact, it was not clear if a favourable surface integrity can be produced in a controlled way using a set of selected process parameters until recently a predictive phenomenological model was presented by Hosseini . The investigations on AISI 52100 have shown that different types of WL can be created that possess significantly different characteristics when one of the two driving forces, excessive heat or plastic deformation, can be isolated. The white layers would then be differentiated into thermally-induced WL (T-WL) and mechanically induced WL (M-WL). Due to their nanocrystalline microstructure (Fig. 1), no or severely reduced retained austenite content, compressive residual stresses and lack of dark layer, M-WLs are expected to have advantageous properties. For them to also have industrial relevance as process-induced functional coatings, the WL would have to endure elevated temperatures as well as external dynamic/static loading as most applications would induce them. However, the intrinsic instability of the nanostructure in the M-WLs may compromise the gain in properties (compressive stresses, hardness, etc.) by the occurrence of grain growth during exposure to elevated temperatures. Thermal stability is therefore a fundamental materials issue for these process-induces functional coatings and the investigation is the prime task in this study, although the applied loads (dynamic/static) will also play an important role in destabilizing the microstructure.
T-WL and M-WLs were investigated in detail by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT). For determining the thermal stability, both types of WL were also investigated after heat treatments at 235, 335 and 435°C using OM and SEM. Furthermore, the hardness of the structures was evaluated at different temperatures.
The microstructural characterization indicated a clear decomposition of the WLs at elevated temperatures, transforming from not being able to resolve any features at all at room temperature and 235°C, to being able to detect features present in the structure after annealing at 335 and 435°C, respectively (compare Fig. 2). At the highest temperature, the primary (Fe, Cr)3C-carbides are more visible, whereas at the lowest temperature even such carbides were difficult to detect using SEM as they are embedded in the WL. Given, the major differences between T-WL and M-WL, the grain growth process will also be significantly different. In T-WLs, Hosseini et al.  reported about 13 vol.% of retained austenite, while in the case of M-WLs, no austenite could be detected. That means that during tempering, in T-WLs the austenite will decompose into ferrite and cementite, whereas in the M-WLs, with increasing tempering temperature, alloy partitioning of e.g. Cr will allow for transformation from the as obtained non-stoichiometric carbides to stoichiometric carbides. Cr, Mn and Mo partitioning inside carbides at temperatures of about 450°C was also reported by Clarke et al. . XRD and TEM analyses are used for a better understanding of the thermal stability of such structures and for determination of the microstructure constituents. The microstructural changes in the WLs are also accompanied by a decrease in hardness with increasing annealing temperature. Our investigation revealed that the M-WL as compared to T-WL possess a ~50 HV higher hardness at 235°C and 335°C. However, at 435°C, the differences are equalized between the WLs.
Summary and conclusion:
· It can clearly be shown that with increasing temperature, a larger portion of the WLs decompose. This occurs independent of if the WLs are mechanically or thermally induced.
· The hardness measurements showed that the M-WL had initially a higher hardness than the T-WL near the machined surface. However, with increasing annealing temperature, the hardness values between them equalized and reached the same hardness.
· Both M-WL and T-WL seem to be stable up to at least 235°C. This is promising for a potential application of M-WLs as functional coatings as temperatures above 200°C are not expected in intended applications.
White layer thermal stability, functional coatings, electron microscopy, atom probe