The microstructure and properties of unbalanced magnetron sputtered CrN[x] coatings.

The most widely used surface treatment to protect engineering components is the deposition of hard chromium by electroplating. The coatings are known to be quite thick (up to 20 mum), reasonably hard (-HV1000), but contain micro-cracks. This wet deposition process is well understood, but it has technical limitations and is under high political pressure because of the environmental pollution by hexavalent chromium. The physical vapour deposition (PVD) technique is an alternative method to produce high quality coatings. PVD is an almost pollution free technique, because the process occurs under vacuum. CrN by PVD is one of the most promising PVD coatings as a candidate to replace eventually electroplated hard chromium. The growth characteristics of CrN coatings are less understood than those of TiN, the well-known PVD coating material. This thesis anticipates to fill this technological gap. Along a wide range of experiments based on the deposition of CrN[x] coatings, XRD, SEM, SNMS and tribological analysis have been used to complete a thorough understanding of CrN[x] growth.The experiments show that there exist several different phases within the Cr-N system: bcc-Cr, hcp-Cr[2]N, fcc-CrN, and mixed phases. This is not fundamentally new, but the work has resulted in two new modifications, which are highly interesting candidates for the industry, including electroplating replacements, namely high nitrogen containing metallic bcc-Cr (solid solution with up to 18 at.% nitrogen) in the hardness range up to HV1800 and a very hard fcc-CrN phase with hardness values between HV1500 and HV3000, similar to TiN.The solid solution bcc-Cr-N is very dense fine-grained, reasonably hard (almost twice as hard as electroplated hard chromium), very smooth, and with a Young's modulus very similar to that of (hardened) steel. The hard fcc-CrN phase (approximately three times harder than electroplated hard chromium) could only be obtained by the current experiments in a rather non-conventional magnetron sputtering parameter window: a combination of a high substrate bias voltage (> -200 V) and a high partial pressure of nitrogen (a multitude of the argon partial pressure). This phase shows a strong {100} preferred crystallographic orientation and shows an excellent behaviour against corrosion and wear.