Nanocrystalline γ-Al2O3 thin film deposited by magnetron sputtering (MS) at low temperature

Abstract

This article presents some experimental data indicating the possibility of depositing nanocrystalline c-Al 2 O 3 thin films on 304L stainless steel, at low temperatures, using magnetron sputtering (MS) in a reactive atmosphere (O 2 ). The experimental conditions are defined, while the coat layer is characterized and first results on its wear behavior are presented.Alumina (Al 2 O 3 ) is one of the most important coating materials used in a wide range of applications due to its excellent mechanical and other physical properties as well as improving the tribological behavior of steel although it is very thin. It can be deposited using chemical vapor deposition (CVD), physical vapor deposition (PVD), pulsed laser deposition (PLD) as well as MS, which produces thin film coating with smooth and dense layer. Different Al 2 O 3 phases can be formed depending on the coating technique and the substrate temperature being the key parameter. The most common and most thermodynamically stable alumina is the polymorphous a-phase. 1-3 It is formed by using PVD at a lowest temperature of 280°C or by using CVD, but at much higher temperatures such as 1000°C. 3-5 It can be deposited also by using reactive MS. 6 Crystalline Al 2 O 3 , frequently deposited, has different forms, namely j-, d-, h-and c-phase. 3,7,8 These phases are metastable, being formed using CVD as well as PVD coating techniques. 6,9 To deposit crystalline alumina phases, substrate temperatures higher than 500°C are generally required. 6,9 The advantage of the polymorphous a-phase as a coating layer is due to its low formation temperature. 5 However, the layer deposited exhibits a rather low hardness of 5-10 GPa, while crystalline phases have higher hardness from 10 to 22 GPa. 10 Therefore, crystalline Al 2 O 3 is considered interesting. However, the high temperature needed for the deposition will cause changes of substrate material properties and coarser grain sizes. 4 Accordingly, a great emphasis over the last ten years has been directed toward developing PVD, in which crystalline films can be deposited at T s lower than 500°C. [1][2][3]6,9,[11][12][13][14] In a recent work, 9 it has been reported that by PVD-pulsed MS, c-Al 2 O 3 was deposited at T s even as low as 290°C. Hence, deposition at a rather low substrate temperature (T s ) is a preferred goal of process development.In the present work, Al 2 O 3 coating was deposited onto polished square pieces (1 9 1 9 1 mm 3 ) of 304L stainless steel substrates using MS. Before deposition, etching by argon plasma was performed in order to clean the metal surface. During the deposition, the stainless steel substrate was heated up to 150°C. Deposition parameters were controlled (Table 1) so that three specimens were realized with 300 nm Al 2 O 3 thickness. The S3 sample conditions were adopted for deposition as reported in Table 2. MS samples (PVD3, PVD4 and PVD5) were prepared at low substrate temperature (150°C) by reactive (O 2 ) direct current sputtering on a planar magnetron.The crystal structure of the films was...

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