DEVELOPMENT OF ANODIZED AI-OXIDE WITH CNT/CNF IN NANOPORES AND ITS TRIBOLOGICAL BEHAVIOUR

Abstract

The replacement of materials in conventional components by lightweight aluminium, its alloys and composites based on these matrices has lead to significant weight saving and consequently, improved fuel economy especially in space, aerospace, automotives and marine industries. In view of depleting fossil oil and its increasing cost, there is a need to pursue the objective of weight saving more aggressively by the materials and design engineers. Aluminium and its alloys have been extensively investigated because of their high strength to weight ratio, good corrosion resistance, and high thermal conductivity. However, these materials have shortcomings including low hardness, high friction coefficient, tendency of galling and difficulty in lubrication, which limit their extended applications. These shortcomings may be overcome by developing self-lubricated, hard and wear resistant coatings of nano-composites which has opened up new possibilities in extending material properties. The basic idea of the present work is to develop an oxide layer over the surface of aluminium of different levels of purity by anodizing process, a traditional and relatively simple way to impart hardness and thus improve wear and corrosion resistance of aluminium and its alloys. But the hard anodic oxide film surface does not provide a low friction lubricating surface. It is fortunate that the anodized surface has porosity and it is possible to generate self organized pores in this oxide layer by appropriate control of the anodization process. The porous anodic alumina films are potentially advantageous as lubricant reservoirs, although they have received only limited preliminary attention in tribology. The pores in the anodic aluminium layer could be impregnated by the lubricant (either in the liquid or solid state or both) and there will be slow dispensing of the lubricant on the sliding surface from the pores, which have very high aspect ratio (depth divided by diameter). iii Invention of Carbon nanotubes (CNTs) in the early nineties has provided the possibility of a graphene based structure which may fit in the pores of anodized alumina. Two step anodization process developed in the mid nineties has resulted in near perfect hexagonally ordered nano-sized pores in anodic alumina layer. These mono-disperse nano-pores could act as templates for aligned growth of CNTs. Thermal chemical vapour deposition (CVD) process is widely used for the growth of CNTs/ Carbon nanofibers (CNFs). These CNTs and/or CNFs have shown low friction because of their graphitic structure and peculiar geometry. However, tribological applications of CNTs are still in its infancy. A few research groups have reported the tribological application of CNTs/CNFs which are limited to low loads primarily for MEMS and NEMS. No studies, to the best of my knowledge, have examined such tailored surface of anodized alumina layers containing CNTs/CNFs in the nano-sized pores for engineering applications at higher loads. In view of the above, the present study involves study of two stage anodization process of pure and commercially pure aluminium for developing self organized nano-sized pores in the porous anodic aluminium oxide (PAAO). CNTs/CNFs have been grown in the pores of these PAAO substrates to result in alumina based composite. The tribological performance of the composite surface of porous anodic aluminium oxide (PAAO) having CNTs/CNFs embedded in it, has been studied under dry sliding condition and also, in presence of lubricant oil with the objectives to develop a low friction wear resistant surface. The effect of process parameters on the outcome has also been investigated for the different stages of the overall process, viz. (i) anodization (formation of porous anodic alumina substrates) and (ii) growth of CNTs/CNFs in the pores of the substrates by thermal CVD. The work in the thesis has been divided in the following seven chapters. Chapter 1 (Introduction) presents a brief overview on the role of aluminium and its alloys in transportation, aerospace sectors and light weight structural components. It iv also provides a brief introduction to the importance of various aluminium based self-lubricating wear resistant coatings including porous anodized aluminium oxide layer embedded with CNTs/CNFs in the pores by CVD. The tribological behaviour of these surfaces has been discussed. Chapter 2 (Literature Review) presents a review of the literature pertinent to all sections of the thesis work. Summaries of the research findings reported by various research groups world wide are presented. This chapter has been subdivided into three sections. Section 2.1 describes the various aspects on preparation of Porous Anodic Aluminium Oxide (PAAO) template as it has been used in the present study to grow Carbon Nanotubes (CNTs) and Carbon Nanofibres (CNFs) in the pores. ' Major emphasis has been given on the optimization of PAAO growth as tribological performance of the coating developed, depends upon the morphology and structure of PAAO. Various aspects of anodization process, including pre-treatments and post-treatments applied have been discussed. Two-step anodization, an effective way for obtaining well ordered porous structure has been described in detail. An overview on effects of various process parameters of anodization on the resulting pore structure, morphology and material properties has been presented. In section 2.2, growth of CNTs/CNFs in PAAO has been discussed. Since present study deals with the cost-effective thermal chemical vapour deposition method to grow CNTs/CNFs, therefore major emphasis has been given to develop self-lubricating coating by this method only. However, other methods for CNT/CNF syntheses have also been discussed in brief The effect of CVD parameters on the resulting CNTs/CNFs structures has also been discussed. Section 2.3 describes tribological studies on the CNTs/CNFs embedded PAAO. It is to mention that very few research results are available in the open literature. Chapter 3 (Experimental) describes experimental procedures used in the present work for developing CNTs/CNFs embedded self-lubricated porous anodic aluminium oxide (PAAO) nano composite surfaces. Details of testing equipments and testing/characterization methods used for the preparation of composite surfaces and their tribological testing are presented. Since morphology and chemical composition play a crucial role in the performance of the nano sized materials, Field Emission Scanning Electron Microscopy (FESEM) coupled with EDAX is used to examine the morphology of the surfaces at all the three stages i.e. development of (i) PAAO layer, (ii) CNT/CNF and (iii) tribo surface characteristics. This chapter has been divided into three different sections. Section 3.1 describes the anodization and pore structure characterization of PAAO developed. Section 3.2 deals with the experimental method (thermal CVD) to grow CNTs/CNFs within the pores of AAO. The CNTs have been characterized by Raman spectroscopy and TEM Section 3.3 describes the tribological testing procedures and associated characterization techniques used in this study. Chapter 4 (Results and discussion: Anodization) outlines the results of the anodization and their explanation. This chapter has been further subdivided into various sections, the results of initial steps, which are common to all processes for anodization, have been presented. The effect of pre-treatment of as-received substrate on the morphology of resultant anodized alumina film and the improvement in the ordering of porous structure in two step anodization compared to one step anodization are reported. The surface roughness of the substrates as estimated by optical profilometer and Atomic Force Microscope (AFM), decreases significantly in pure as well as in commercial aluminium samples by electropolishing. Pore ordering has been found to improve if samples are pretreated by annealing than electropolishing. However, the samples with joint electropolishing and annealing show still better ordering. The current density through the sample increases exponentially with voltage applied. The porosity that develops in the PAAO decreases as the anodization voltage increases. The effects of process variables such as applied voltage, current, temperature, concentration and pH of the electrolyte, on the resulting anodized alumina structure for acid electrolytes, are presented in this chapter. The results of anodization i.e. ordering and uniformity of the porous vi structure obtained by phosphoric acid, oxalic acid, chromic acid and sulphuric acid electrolytes are presented, which have further been subdivided into two sections in which the structural features of the grown anodized layer of pure aluminium and commercial aluminium samples have been reported. `Self ordering regime', is a set of anodization parameters for a given electrolyte, which provides well—ordered porous structure. Such regime has also been established for phosphoric and oxalic acid electrolytes in the present study. The time duration in the first step of anodization also plays a key role in the degree of ordering. Longer the time duration, more is the degree of ordering. On the basis of characteristics of the porous alumina films, it can be concluded that 1. The pore diameter and interpore distance are linearly dependent on the applied voltage. 2. The degree of ordering of the porous film morphology is a function of time duration of anodization in the first step of the standard 'two-step anodization process'. 3 The self-ordering voltage varies with the electrolyte used. Electrolytes which have higher dissolution ability of aluminum have lower self-ordering voltage. 4. There is no significant change in the results of anodization on pure aluminium and commercially pure aluminium substrates. However, pure aluminium is slightly better in terms of regularity and ordering of porous structure. Chapter 5 (Results and discussion: CNTs/CNFs growth within porous Alumina) provides the results of the experiments carried out to establish influence of PAAO templates and that of CVD process parameters on the growth and morphology of resulting CNTs/CNFs embedded PAAO. This chapter has been divided in six sections. Section 5.1 provides common findings of growth evidence of CNTs/CNFs in the pores through cross sectional view of CNT embedded samples and EDAX analysis for a few representative samples. Section 5.2 provides influence of pore diameter of the PAAO template on the diameter of the CNTs/CNFs produced by CVD. Section 5.3 describes effects of vii catalyst and the catalyst deposition process on the resulting morphology and structure of the CNTs/CNFs developed. Three subsections of 5.3.1, 5.3.2 and 5.3.3 explain the influences of catalyst deposition process, the type of the catalyst and PAAO template acting as catalyst, respectively on the morphology of CNTs/CNFs. Sections 5.4 and 5.5 provide results of influence of time and decomposition temperature of CVD on CNTs/CNFs structure and morphology. On the basis of the results obtained, following inferences have been drawn. (a) The diameter of the PAAO template plays a major role on the growth of CNTs/CNFs within the pores, especially from the bottom of the pores. (b) For pore size less than 100 nm, CNTs/CNFs grow from the top of the PAAO, rather than from the pore bottom. (c) Catalyst deposition is an important step in the synthesis process as improper deposition conditions lead to defective structures in CNTs / CNFs. (d) Type of the catalyst influences the growth, structure and morphology of the CNTs/CNFs. When cobalt is used as catalyst, a good yield of tubular structures is obtained. CNTs/CNFs are found to grow from the bottom of the pores and such growth scheme is found applicable in majority of the pores. (e) The CNTs characterization by Raman spectroscopy shows that CNTs with highest degree of graphitization can be obtained using Lithium compound catalyst used. The catalysts Cobalt, Nickel and Nickel-Cobalt compound used result in decreasing order of graphitization in the CNTs developed. (f) CNTs growth on PAAO substrate mainly from the top surfaces has been observed without any catalyst used, however the growth in this case is non-uniform and sparsely distributed. (g) The optimum time required for the growth of CNTs at 650 °C is 10 minutes. The carbon structures other than tubular structure (CNTs and CNFs) are found to develop for growth time more than 10 mins. (h) When CVD temperature is kept at 500° C, no growth of CNTs has been observed except at few points. viii (i) The average diameter of the CNTs/CNFs has been found to increase exponentially up to a certain limit of temperature. (j) There are other factors which also affect growth of CNTs/CNFs by CVD in PAAO and therefore the synthesis is a complex process. The chapter ends with a discussion on all of the above mentioned results. Chapter 6 (Results and discussion: Dry and lubricated friction and wear) presents the results of tribological studies on the composite surface of PAAO embedded with CNTs/ CNFs. Dry sliding friction and wear behaviour have been studied in a pin on disc set-up against the counterface of hardened steel under different normal loads and fixed sliding speed. The results have been compared with that of the PAAO alone without any CNTs/ CNFs in the pores. In addition, wear tests on the composite surface has been carried out in presence of lubricating oil and the results have been compared with the corresponding results for dry sliding. The pores in PAAO surface have been sealed and subjected to wear tests both under dry and lubricated conditions, for comparison. The following are the findings: (i) CNTs/CNFs embedded PAAO results in reduction in both wear rate and friction forces compared to that of PAAO alone, tested under dry sliding condition. (ii) The wear and friction on the surface of PAAO alone and that with CNTs/CNFs embedded in pores decrease markedly in presence of lubricating oil over those observed under dry sliding condition. (iii) Wear resistance of PAAO surface anodized in oxalic acid is generally more than that of the surface anodized in phosphoric acid bath. However, the friction coefficient is significantly lower on the surface anodized in phosphoric acid bath as compared to that on the surface anodized in the oxalic acid bath but there is a trend reversal for friction in presence of lubricating oil i.e. friction is less on surface anodized in oxalic acid bath compared to that anodized in phosphoric acid bath. These results are followed by discussion