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由钢内氢扩散引发的氢脆现象。氢扩散的一个重要途径是其从润滑剂分解产物中渗入钢基体。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渗入轴承钢的过程进行了定量研究。研究中使用氘代润滑剂，以区分钢中原有的氢与润滑剂分解产生的氢。氘的浓度始终在滚道表面附近达到峰值，并随深度增加逐渐降低。扩散进入的氘含量随摩擦时长呈抛物线关系增长。为了防止钢基体发生氢渗入，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。类金刚石碳涂层可减少但无法完全阻止氢渗入，其作用机制是降低了润滑剂在摩擦化学与热分解过程中产生的氢总量。