Effect of Diamond-Like Carbon Coating on Preventing the Ingress of Hydrogen into Bearing Steel

The failure of moving mechanical components is often accompanied by hydrogen embrittlement caused by hydrogen diffusion in steel. One considerable mechanism for hydrogen diffusion is its ingress from lubricant decomposition. We quantitatively investigated the ingress of hydrogen generated by lubricant decomposition into bearing steels using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Deuterated lubricants were used to discriminate between the original hydrogen in steel and the hydrogen generated from lubricant decomposition. The highest concentration of deuterium was always near the raceway surface, and the concentration decreased gradually as the depth was increased. The diffused deuterium increased in a parabolic relation with the duration of friction. To protect steels against the ingress of hydrogen, we studied the effect of a diamond-like carbon (DLC) coating, which has a low diffusion coefficient of hydrogen. DLC reduced but did not completely prevent the ingress of hydrogen, which resulted from the reduction in the amount of hydrogen generated from the tribochemical and thermal decomposition of lubricants.运动机械部件的失效往往伴随钢材内部氢扩散引发的氢脆现象。氢扩散的重要途径之一，是氢从润滑剂分解产物中侵入钢材内部。本研究采用飞行时间二次离子质谱（time-of-flight secondary ion mass spectrometry，ToF-SIMS），对润滑剂分解产生的氢侵入轴承钢的过程开展了定量分析。实验中使用氘代润滑剂，以区分钢材中原有的氢与润滑剂分解生成的氢。结果显示，氘的最高浓度始终位于滚道表面附近，且随深度增加浓度逐渐降低；扩散进入钢材的氘含量随摩擦时长呈抛物线型增长。为防护钢材免受氢侵入，本研究探究了氢扩散系数较低的类金刚石碳（diamond-like carbon，DLC）涂层的防护效果。结果表明，DLC虽可减少氢侵入，但无法完全阻断该过程，其作用机制在于降低了润滑剂经摩擦化学分解与热分解产生的氢的总量。