research papers 1360 M. Chis et al.  Hardness of oxide films J. Appl. Cryst. (2000). 33, 1360±1364 Journal of Applied Crystallography ISSN 0021-8898 Received 20 April 2000 Accepted 29 August 2000 # 2000 International Union of Crystallography Printed in Great Britain ± all rights reserved Hardness of oxide films formed as a result of aluminium anode oxidation processes M. Chis, a ² M. O. Cojocaru, b D. Cojocaru b and R. A. Palmer a * a Department of Crystallography, Birkbeck College, University of London, Malet Street, London WC1E 7HX, England, and b University Politehnica Bucharest, Materials Science and Engineering Faculty, Romania. Correspondence e-mail: r.palmer@bbk.ac.uk Factors that in¯uence the anode oxidation processes of aluminium and its alloys are discussed. The main parameters involved in such processes have been selected and subjected to measurement under controlled experimental conditions. The effects of the variation of the chemical, electrical and thermal parameters of the aluminium anode oxidation process on the hardness level of the resulting oxide layer have been studied. In order to quantify the relationships that govern the kinetics of the anode oxidation process, these data were subjected to analysis employing a central compositional rotatory programming matrix. Parameters employed were electrolyte concentration, temperature, current density and immersion time of the product in the electrolyte to achieve a desired oxide layer. The hardness of the resulting oxide layers is discussed. 1. Introduction Developments in many technical ®elds involve intensive utilization of aluminium-based light alloys. Aluminium-based alloys have made a signi®cant impact over a wide spectrum of industrial activities, including the car industry, the chemical industry, electrotechnical aeronautics, marine applications, the food industry, and in highly innovative ®elds such as astro- nautics. In many important practical situations, the value of the main physical±chemical and mechanical characteristics of aluminium and its alloys can be decreased by their tendency to oxidize under the in¯uence of environment conditions. The destruction of aluminium or its alloys by physical±chemical interaction with the environment manifests itself as a non- uniform oxidation, which occurs either with linear or parabolic kinetics. Paradoxically, one of the main methods of combating the oxidation of aluminium and its alloys involves the oxida- tion itself. The process involved is both arti®cial and rigorously controlled, and provides the desired physical±chemical and mechanical characteristics in the super®cial layers of alumi- nium or its alloys by causing changes in the chemical composition of these phases. During the controlled oxidation process, the super®cial metallic layer undergoes an in situ transformation into a ®lm composed mainly of the base-metal compounds with oxygen. The oxide ®lms thus formed have thickness greater than the wavelength of light (i.e. > 4000 A Ê ) and are visible to the unaided eye. The characteristics of such layers of aluminium oxides are strictly dependent upon the conditions prevalent during the oxidation process and during any subsequent thermal process. In the early stages of the anode oxidation process (Mondolfo, 1979), amorphous aluminium oxide appears in the super®cial layers of aluminium or its alloys. Over a period of time, in such layers the aluminium and oxygen atoms adopt a de®nite arrangement, with O 2 ions forming a compact cubic arrangement and with Al 3+ ions ®lling the octahedral spaces of the lattice, corresponding to the well known structure of  - Al 2 O 3 . The  -Al 2 O 3 modi®cation formed by low-temperature oxidation (<1200 K) has a cubic lattice with a = 7.90 A Ê , density = 3.6 g cm 3 (Kalinina, 1959) and a melting temperature (2250 K) which is much higher than that of pure aluminium (933 K) and very close to that of the high-temperature stable form of aluminium oxide, -Al 2 O 3 (corundum) [T melting - Al 2 O 3 = 2310±2320 K (Ruff, 1978; Morey-Bull, 1934; Geller & Yavorsky, 1945; Carruccini, 1949; McNally, 1961)]. The extreme hardness of this material [2000±21000 MN m 2 (Filomenco et al. , 1957)] further justi®es the interest shown in the presence of such an oxide in the super®cial layer of aluminium or aluminium-based alloys. By using boric acid or borate-based solutions (Dekker, 1978a,b) as electrolytes in the process of anode oxidation, or by employing voltages higher than 100 V,  -Al 2 O 3 can be obtained without initiating the formation of amorphous aluminium oxide in the early stages (Taylor, 1978a,b). The presence of magnesium in low amounts (several parts per million), promotes the germination of  - Al 2 O 3 (Dignam, 1978), while the presence of copper decreases this tendency. The stable -Al 2 O 3 modi®cation frequently occurs as a result of the oxidation of aluminium at temperatures higher than 200 K (Yamaguchi, 1978a,b; Phelps, 1978; Tajima, 1978), the transformation of the  -Al 2 O 3 state (at temperatures ² Senior Surveyor at Lloyd's Register, UK.