Tribological Studies of Advanced Materials for Aerospace Braking Applications
The most important and basic objective of aircraft operation is safe flying, which is achieved through proper functioning of various components. Amongst these components, lot of importance has been given to the tribological properties of brakes in an aircraft, as its failure could lead to catastrophic possibilities.
The materials employed for these braking applications must satisfy a range of requirements like high stiffness, thermal and chemical stability, and most importantly deliver stable friction coefficient and high wear resistance. Amidst ever developing technologies, the need of advanced materials to provide the required and desired mechanical, environmental and stable combination, which is expected off a braking system, is gaining dominance. Some of the best possible combination available have their own disadvantages and for now, it has become essential for researches to try to find a variation in the brake composition for currently available materials.
The objective of the present doctoral research was to study the tribological properties of different combinations of materials (typically disc and friction pad materials) intended for aerospace applications. Further, the project aims, and structure were to identify the best frictional couple through the tribological study of existing materials and the development of new ones, develop a methodology to investigate friction and wear mechanisms, and lastly to develop a map of tribological properties of material couples. The materials (and combinations) currently being employed or researched for aerospace braking applications are Cu based materials with heat treated steel, C/SiC composites, C/C composites and HVOF coatings.
In this current study different categories of friction materials were paired with distinct classes of counterface to evaluate the tribological characteristics of these combinations/pairings. The friction materials tested were Cu-Sn based, Cu-W based and SiC-graphite composite materials. The Cu based composite materials were selected for their properties like thermal and electrical conductivity, resistance to oxidation, and stability at extreme conditions. On the other hand, the SiC-graphite composite material was selected due to its low density, prolonged service life, high thermal shock resistance, and low property fluctuations to temperature and surroundings. Two different kinds of counterface were selected for pairings and subsequent testing. The first category of counterface consists of three types of steel: high Cr – low C (APX4), low alloyed (AISI4140), and martensitic stainless steel (AISI420c). The second category was of two types of high velocity oxy fuel (HVOF) cermet coatings. Cermets coatings are typically wear resistant materials that possesses a combination of high toughness and hardness. Cermets provide for superior hardness from ceramic materials and toughness from metal. Two different cermet coatings were analyzed – WC-CoCr and WC-FeCrAlY coating. The composite materials were tested in the form of pins of height between 12 – 12.5 mm and diameter 10 mm and the counterface were tested in the form of discs of diameter 100 mm and thickness 7 mm. The pins and discs were tested on a pin on disc testing equipment. The pin on disc equipment provided for the trends and magnitude of friction coefficient, pin and disc wear. The mated surfaces were further subjected to SEM/EDS/XRD to understand the composition, extension, bonding, and nature of the friction layer deposited on the mated surfaces.
The studies can be divided into three parts. The first part is focused on the analysis of the Cu-Sn and Cu-W based composite material with different grades of steel. The Cu-Sn composite material named SPEC 283 was paired with all the three types of steel counterface. The room temperature analysis (RT) was conducted at 0.5 and 1 MPa, 1.57 and 7 m/s, for a period of one hour. At 1.57 m/s, the most notable result was the highest pin wear recorded with APX4, which was in the severe region (above 10-13 m2 /N). All other steel observed wear between mild and severe. At 7 m/s, the significant result was with respect to a particularly high COF for AISI420c at 0.5 MPa. The friction layer at all the testing conditions was made predominantly of Cu and Fe oxides with difference in extension at different velocities and pressure. Nevertheless, the major outcome of this part of study was the discontinuation of tests with APX4, due to its high wear characteristics. The high temperature tests (HT) on SPEC 283 was conducted at 400°C, with AISI4140 and at same velocities and pressure as RT tests. The focus was on the major composition and phases of different constituents of friction layer on the pin and disc surfaces. At HT conditions, the pin wear was well in the mild and bordering to ‘very mild’ range. With respect to this, the major observation recorded was the importance of a well-established friction layer on the interfaces that led to superior tribological characteristics. The tests with Cu-W based composite material titled 7387-64 was conducted with AISI420c at 0.5 MPa, 1.57, 4.5 and 7 m/s, RT and HT conditions. Also, in this case, the pin friction layer was made of Cu and Fe oxides (transferred from disc surface). The results with 7387-64 were not as satisfactory as that of SPEC 283 due to the low deformability of the matrix, accompanied by the presence of NbC in the composite material, which restricted the spread of the friction layer, thereby causing comparatively elevated pin wear and undesirable magnitude of friction coefficient.
The second part of the studies was fully focused on testing the SiC-graphite composite material, named AS01, with both the classes of counterface. Firstly, AS01 was tested with the AISI420c disc at 0.5 MPa, 1.57 and 7 m/s and at a range of high temperature starting from RT, 100°C, 200°C, 250°C, 300°C, till 400°C. The major observation with respect to AS01 paired with AISI420c was the severe wear observed in the case of all the tests with 1.57 m/s and mild wear recorded for all tests at 7 m/s. However, when the AS01 testing method/procedure was tweaked, it revealed that as and when AS01 already had a preexisting friction layer on its surface, the material would perform exceptionally well even at low velocities and high temperatures. The tests with the coated discs were divided into two groups – RT and HT conditions. The RT tests were conducted at 0.5 and 1 MPa, 1.57 m/s, and both with AS01 and SPEC 283. The results of the RT tests were compared with the results of AS01 and SPEC 283 with AISI420c. It was seen that the coated disc displayed comparatively high friction coefficient and low pin wear compared to AISI420c. The HT tests were conducted only on AS01 at 300°C, 0.5 and 1 MPa and 1.57 and 7 m/s. From the tests at HT conditions it was seen that AS01 was able to form a highly compact and extended friction layer with extremely low pin wear, even at low velocity and high temperature, when compared to the uncoated AISI420c counterface. In all the cases, the friction layer on the mated surfaces predominantly constituted of Fe oxides. Further emission analysis on WC-CoCr coating and AISI420c counterface were conducted with AS01 at 0.5 MPa, 7 m/s RT. Also, in this case, the coated disc gave rise to appreciably low emissions when compared to AISI420c counterface. Hence, from the second part of the studies, a favorable relationship of AS01 with friction layer as well as cermet coatings were observed and clearly appreciated.
In all the cases with SPEC 283, 7387-64 and AS01, it was seen that the friction layer was predominantly made of Fe oxides, which were transferred from the counterface surface. Hence, it piqued the curiosity to observe the effects on tribological properties of wear systems with considerable Fe content present in the friction material. Hence, the third part of the doctoral studies was focused on the study of different classes of composite materials containing varying Cu and Fe content. This section of the research was divided into two sub parts. The first sub part focused on the starting composition and production route of these composite materials. The starting composition had to be selected from three variation – first replicating SPEC 283 (OC), second from literature (LSC), and third without silica (WSC). The third variation was made after seeing that the major difference in the composition between first and second variation was the difference in silica content. The production route had to be selected between spark plasma sintering (SPS) and conventional sintering technique. Due to the desirable combination of friction coefficient and low pin wear, LSC was selected as the best starting composition. On the other hand, the SPS method was selected as the production route due to its ability to produce extremely dense and robust specimens compared to conventional sintering technique. The second sub part constitute of producing two classes of composite materials with varying Fe and Cu content starting from LSC composition and SPS technique. The first class of composite material constituted of specimens containing increasing Fe content and decreasing Cu content. The second class of composite material constituted of specimens with Cu fully eliminated and decreasing Fe content. From the preliminary examinations conducted on both the classes of composite materials, it was seen that the Fe addition in the composite materials worked favorably towards tribological characteristics. However, due to the brittle nature of the second class of composite materials containing less Fe and evolution of high system temperature during testing, due to no Cu content, led to the conclusion that optimized amount of silica and Cu are necessary for elevated performance of specimens and wear systems.
Organization of the thesis: Chapter 1 deals with the basic concepts on friction and types of wear mechanisms. Also presented are information and mechanism of aircraft braking with supplementary diagrams and a list of materials being employed for braking applications. Chapter 2 explains the general friction and wear behavior of different types of friction materials – Cu based and SiC-graphite composite materials. Additional information has also been provided on the effects of different types of lubricants and abrasives on the wear characteristics of a composite materials and ultimately the system. Chapter 3 predominantly explains the selection of various grades of steel and cermet coatings as counterface for this study, through explanations on the respective constituents and wear mechanisms of the materials. Chapter 4 is a short and basic introduction to spark plasma sintering (SPS) process, which has been predominantly in the third part of this doctoral research. Chapter 5 deals with the materials and methodology employed in this study. The materials part explains the composition and constituents of all the friction materials and counterface. The methodology portion focuses on the testing conditions and durations employed for different pairing, and different equipment (with pictures) used during studies. Chapter 6 delves into the extensive study of SPEC 283 specimen, divided into two parts – RT and HT conditions. Chapter 7 focuses on the studies of 7387-64 at different testing conditions and its subsequent variations in testing methods and wear mechanisms.
Chapter 8 explains the various characteristics portrayed by AS01 material at different elevated temperature and velocities and possible fix that could assist AS01 to operate desirably at all testing conditions. Chapter 9 extensively explains the behavior of both cermet coatings with different friction materials, at different testing conditions, its subsequent wear mechanism and friction layer characteristics. Further, a basic emission evaluation and comparison of the cermet coating is also mentioned with preliminary analysis. The final part, Chapter 10 deals with the third part of the study, to evaluate different classes of composite materials with varying Cu and Fe content. The chapter is broadly divided into two parts – the first part deals with the selection of the starting composition and production route and the second part actually deals with the preliminary evaluation of the two classes of composite materials.
Below is a simple flowchart of the overview of the studies conducted in this thesis. The big rectangle denotes the collection of observations of the studies of all the materials inside the box, which led to the conception and analysis of Cu/Fe based composite materials.