Diamond challenged by hard materials: a reflection on developments in the last decades

This paper reviews the status of chemical vapor deposition (CVD) of diamond. The CVD process has made enormous steps forward contrasted to high pressure/high temperature (HP/HT) synthesis. Fabrication of coatings and free standing diamond plates exceeding 10 cm has become a reality. But perseverance is necessary in solving many difficult problems, which have created barriers in widening diamond applications. As CVD and HP/HT research proceeded, a challenge came from suggestions that “harder than diamond” β-C3N4 material can be predicted from first principles calculations. A more recent claim “diamond hardness achieved” considers nanograin hardening in composite material films TiN/SiNx. These results and suggestions have kept a high level of research activity around hard materials and at the same time disclosed difficulties in understanding and measuring hardness.