JOURNAL OF MATERIAL SCIENCE 30 (1995) 4479-4482 Nanoporosity of AI203 coatings obtained by impulse plasma deposition K. ZDUNEK Department of Materials Science, Warsaw University of Technology, 85 Narbutta, 02-524 Warsaw, Poland H. GRIGORIEW Institute of Nuclear Chemistry and Technology, 16 Dorodna, 03-195 Warsaw, Poland Investigations of the size distribution of nanopores were carried out for alumina coatings deposited by the impulse plasma method. The single-phase (metastable and stable correspondingly) alumina coatings were deposited under different conditions of impulse plasma processes. The investigation of nanopore distribution was carried out using small-angle X-ray scattering. Despite the different phase composition of the coatings obtained, the most probable value of nanopores for both alumina coating materials were practically the same and equal to 5 nm. It appears that a coating porosity of the order of nanometres is characteristic for all coatings deposited by the impulse plasma method, because previously similar dimensions of nanopores were found for diamond, TiN and BN. It seemsthat during the impulse plasma deposition the coating grows on the substrate surface by condensation of ultra-small particles nucleated in the impulse plasma. 1. Introduction Aluminium oxide is an electrical insulator, which is hard and exceptionally resistant to chemical agents, and shows a good heat conductivity [1, 2]. As a coat- ing material, it seems to be a prospective candidate for many engineering applications and is especially suit- able wherever the surfaces involved should show an increased mechanical strength and chemical resist- ance, in particular at elevated temperatures, and where heat should be quickly removed. Aluminium oxide is also used as an electrically insulating layer in multi-layer semiconductor devices [3-10]. A120 3 coatings may be produced using either the physical (PVD) or chemical (CVD) vapour deposition method [11-13]. Recently, the impulse plasma deposition (IPD or IPPVD) method has been used for obtaining alumi- nium oxide coatings [14]. This method uses a strongly non-isothermal impulse plasma which is almost fully ionized (lifetime of 10 -4 s). The plasma is generated in a coaxial accelerator which operates in a quasi-sta- tionary mode [15, 16]. A characteristic feature of IPD coatings is their ultrafine-grained structure [17 19]. They adhere well to the substrate even though no external source heats it during the plasma process (the substrate surface is heated only by the impulse plasma [19]). So far we have accumulated a thorough know- ledge of the fundamental and practical problems asso- ciated with coatings made of diamond [18], titanium nitride [17] and multicomponent metallic alloys [19]. We have also studied some aspects of the mechanism of A1203 synthesis [14]. We found that the AlzO3 coatings were built up from ultra-small particles of metastable alumina phases, which were crystallized in the very plasma. According to the IPD process para- meters, we are able to produce alumina coatings with different numbers of macrodefects; in some cases it is possible to obtain very dense coatings even without characteristic grain boundaries [14, 20] (this corres- ponds to the "freezing zone" concept proposed by Zdunek [19]). The aim of the present study was to find a more precise answer to the problem of the condensation mechanism of alumina coatings obtained by the IPD method. In our opinion the condensation mechanism can be best studied by the small angle X-ray scattering (SAXS) method. This is because this method makes it possible to determine the distribution of nanopores existing in the material. 2. Experimental procedure 2.1. Condensation of AI203 coatings The synthesis of AlzO3 coatings took place in the apparatus described in detail elsewhere (e.g. [16, 21]). In this apparatus the source of the vapours is the front face of the end of an internal electrode of the plasma accelerator [16]; the gaseous source of the vapours could be the plasma gas, which is supplied to the plasma accelerator in a continuous way. Taking this into account we have chosen the following vapour sources. 0022-2461 9 1995 Chapman & Hall 4479