Reduction of residual stresses in CVD diamond layers on steel parts through creation of hierarchically structured surface topographies by applying cutting processes

CVD金刚石涂层在钢材料上的应用使其性能得到了各种改进，例如在摩擦机械高负荷系统中减少摩擦和磨损，或在热机械应用中减少功能表面的粘附倾向。由于钢和金刚石的热膨胀系数不同，在从涂层温度冷却到室温的过程中和冷却后，金刚石层中存在较大的残余压应力。因此，CVD金刚石涂层被限制在一个非常小的厚度，因为较厚的层容易脱落。对金刚石涂层钢基体的冷却特性进行了有限元分析，并利用RAMAN光谱对沉积在X46Cr13钢基体上的CVD金刚石层进行了残余应力测量，有助于加深对附着机理的理解。结果表明，超声振动叠加面铣削所产生的预先确定的基板表面微观结构对层内残余应力的分布有显著影响。结合TiNB中间层，实现了定制化接口。这使得厚度高达16微米的封闭、均匀和粘附的CVD金刚石层的应用成为可能。在低于400℃的温度下，奥氏体-贝氏体相变过程中，钢的体积显著增加，从而降低了冷却残余应力，从而使金刚石层具有较好的粘附强度。综上所述，在项目的第一阶段，成功地开发了科学基础，这是通过适当的相变提高CVD金刚石层在钢基体上的粘附强度的先决条件。在此基础上，研究了热质量较高且由“贝氏体起始温度较高”钢（X40CrMoV5-1）组成的CVD金刚石涂层试样的功能特性，该试样具有较强的冷却残余应力。由于转变行为对金刚石层的应力降低不利，这种钢有资格证明和分析表面微观结构对金刚石层粘附力的影响。这将扩大对粘附机制的基本理解以及潜在的应用，因为这种钢也具有很高的技术相关性。由于其特殊的振动特性，它可以用作超声电极材料，例如用于铝部件的超声波焊接。

The application of CVD diamond coatings on steel materials enables various improvements of the performance, such as a reduction of friction and wear in tribomechanically highly loaded systems or a decrease of the adhesion tendency of functional surfaces in thermomechanical applications. The different coefficients of thermal expansion of steel and diamond result in strong residual compressive stresses in the diamond layer during and after cooling down from the coating temperature to room temperature. Consequently, CVD diamond coatings were limited to a very small thickness, since thicker layers tend to flake off. FE analyses of the cooling characteristics of the diamond coated steel substrate and residual stress measurements by RAMAN spectroscopy of the deposited CVD diamond layers on substrates of the steel X46Cr13 contributed to an enhanced understanding of the adhesion mechanisms. It could be determined that a predefined substrate surface microstructure generated by ultrasonic vibration superimposed face milling significantly influences the distribution of the layer-inherent residual stresses. Combined with the TiNB intermediate layer, a tailored interface was realised. This enabled the application of a closed and homogeneous as well asadherent CVD diamond layer with a thickness of up to 16 µm. The relatively good adhesion strength of the diamond layers is mainly attributed to a reduction of the cooling residual stresses due to the significant steel volume increase during the austenite-bainite transformation at temperatures below 400 °C. Summing up, in the first periode of the project, the scientific fundamentals were successfully developed as a prerequisite for increasing the adhesion strength of CVD diamond layers on a steel substrate with an appropriate phase transformation. Based on this, the functional properties of CVD diamond coated specimens with a higher thermal mass and comprising of a "less favourably (higher bainite starting temperature) transforming” steel (X40CrMoV5-1) resulting in stronger cooling residual stresses is investigated. Due to the transformation behaviour which is less auspicious for a stress reduction in the diamond layer, this steel qualifies to demonstrate and analyse the influence of surface microstructures on the adhesion of the diamond layer. This will broaden the basic understanding of the adhesion mechanisms as well as the potential applications, since this steel is also of high technical relevance. Due to its special vibrational properties, it can be used, as a sonotrode material, e. g. for ultrasonic welding of aluminium components.