Abstract
• AlCr(Si)N coatings with an Al/Cr ratio of 70/30 and 0, 2.5 and 5 at.% Si were deposited • The oxidation behaviour up to 1460 °C was studied • Coatings were analysed by advanced cross-sectional X-ray nanodiffraction • The onset of oxidation was increased by the addition of Si from 1100 °C to 1260 °C • Complex elemental and micro-structural variations after oxidation were characterized The resistance of wear protective coatings against oxidation is crucial for their use at high temperatures. Here, three nanocomposite AlCr(Si)N coatings with a fixed Al/Cr atomic ratio of 70/30 and a varying Si-content of 0 at.%, 2.5 at.% and 5 at.% were analyzed by differential scanning calorimetry, thermogravimetric analysis and X-ray in order to understand the oxidation behavior depending on their Si-content. Additionally, a partially oxidized AlCrSiN coating with 5 at.% Si on a sapphire substrate was studied across the coating thickness by depth-resolved cross-sectional X-ray nanodiffraction and scanning trans-mission electron microscopy to investigate the elemental composition, morphology, phases and residual stress evolution of the oxide scale and the non-oxidized coating underneath. The results reveal enhanced oxidation properties of the AlCr(Si)N coatings with increasing Si-content, as demonstrated by a retarded onset of oxidation to higher temperatures from 1100 °C for AlCrN to 1260 °C for the Si-containing coatings and a simultaneous deceleration of the oxidation process. After annealing of the AlCrSiN sample with 5 at.% Si at an extraordinary high temperature of 1400 °C for 60 min in ambient air, three zones developed throughout the coating strongly differing in their composition and structure: (i) a dense oxide layer comprising an Al-rich and a Cr-rich zone formed at the very top, followed by (ii) a fine-grained transition zone with incomplete oxidation and (iii) a non-oxidized zone with a porous structure. The varying elemental composition of these zones is furthermore accompanied by micro-structural variations and a complex residual stress development revealed by cross-sectional X-ray nanodiffraction. The results provide a deeper understanding of the oxidation behavior of AlCr(Si)N coatings depending on their Si-content and the associated elemental, microstructural and residual stress evolution during high-temperature oxidation.
Highlights
Ceramic hard coatings are widely used in industry to protect the surface of tools in various machining and forming applications
The elemental composition of the coatings in the as-deposited state obtained by Elastic Recoil Detection Analysis (ERDA) is reported already in [25] and given in Table 1 for completeness
Referring to the coatings investigated in this work, the onset of oxidation is shifted to higher temperatures from 1100°C to 1260°C for the Si-containing coatings compared to AlCrN, and the oxidation progress in the temperature range from 1200°C to 1460°C is slowed down significantly (Fig. 2 and Fig. 3)
Summary
Ceramic hard coatings are widely used in industry to protect the surface of tools in various machining and forming applications. Ternary transition metal nitrides find widespread use due to their good mechanical properties, thermal stability and oxidation resistance. Modern applications like high-speed cutting or dry cutting demand for wear-protective coatings resisting high temper-. Al(Cr)N and a Cr-enriched cubic Cr(Al)N phase [7]. In another step, c-Cr(Al)N decomposes into Cr2N (and AlN) and Cr, accompanied by the release of nitrogen yielding a porous structure with undesirable mechanical properties [7, 8]. Several attempts have been made to stabilize the AlCrN system in its cubic structure at high temperatures by alloying [9, 10], applying a multi-layered structure [11, 12] or using a dedicated residual stress design [13, 14 ]
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