Cast Iron Counterface Research Articles

A novel path towards limiting non-exhaust particulate matter emissions of a commercial friction material through the addition of metallurgical slag

Keeping recycling and the circular economy in mind, this study explores the reduction in emission of a highly optimized, commercially employed friction material formulation through the addition of metallurgical slags from a basic oxygen furnace in varying quantities from 6 to 38 wt%. The various compositions were paired with a pearlitic grey cast iron counterface and tested on a pin on disc tribometer. The friction coefficient and pin wear increased with the slag addition but were still within the permissible limit when compared to the original formulation. Specimens with higher slag content observed extremely compacted and extended secondary contact plateaus, which also recorded significant slag presence. The extended plateaus detached in the form of chunks from the mating surfaces, which settled on the equipment hardware and restricted the production of airborne particles. From an industrial symbiosis perspective, the addition of metallurgical slags proved to be a promising way of obtaining green friction materials with reduced emissions.

Comparative Studies on the Dry Sliding Behavior of a Low-Metallic Friction Material with the Addition of Graphite and Exfoliated g-C3N4

This study compares the effect of the addition of two types of lubricants on the dry sliding behavior of a simplified Cu-free phenolic resin-based composite material. The lubricants were commercial graphite and exfoliated graphitic carbon nitride (codenamed: TEX6). The graphite particles were rounded and of ‘flaky’ character. The TEX6 particles were not only flaky, but also irregular in shape, and ‘fluffy’. Both lubricants were added individually in the basic formulation and subjected to dry sliding tests on pin-on-disc testing equipment in mild conditions and against a grey cast-iron counterface. The tests with TEX6 observed a stable steady state in the friction coefficient (CoF) with lower scatter and lower average friction coefficient and pin wear magnitude when compared to samples containing graphite. Additionally, the worn surfaces of the TEX6-containing samples had extremely smooth, compact, and continuous secondary plateau coverage when compared to the graphite-containing samples. The counterface paired with the TEX6-containing samples observed much lower abrasive action compared to the graphite-containing samples. Through the wear testing and further evaluation of the secondary plateaus, the possible addition of TEX6 as a lubricant in friction material composition was explained, making it a promising component for automotive braking applications.

A critical comparison of dynamometer data with pin-on-disc data for the same two friction material pairs – A case study

Friction and wear behavior of brake materials is often investigated using Pin-on-Disc (PoD) or dynamometer tests. Dynamometer tests reproduce the real braking conditions since they are carried out on real parts. whereas PoD tests are carried out using small pins, under constant sliding velocity and contact pressure. In the present technical note, the friction and wear behavior of two commercial friction materials, a low-met and a non-asbestos organic (NAO) material, dry sliding against a pearlitic cast iron counterface disc was investigated. The PoD tests were carried out at room temperature and at 300 °C; the dynamometer tests were conducted simulating urban braking conditions. The experimental data were compared and the main wear mechanisms were identified. The wear mechanisms observed with the PoD at room temperature turned out to be similar to those observed with the dynamometer tests. On the other hand, the PoD tests carried out at 300 °C revealed particular features in the friction layer that were not detected in the dynamometric tests. As far as the tribological data are concerned, the average friction coefficients recorded during the dynamometric tests were similar to the values recorded in the PoD tests, whereas the wear coefficients were one order of magnitude lower, most probably due to the limited area of contact of the pins.

A preliminary investigation on the use of the pin-on-disc test to simulate off-brake friction and wear characteristics of friction materials

During off-brake driving, some unwanted contact between the brake pads and the rotating discs may occur, thus inducing inefficient fuel usage and additional environmental pollution. This paper addresses the frictional behavior of 5 commercial low-met and NAO friction materials dry sliding against a cast iron counterface disc, under low contact pressures, typical of off-brake driving. In the investigated conditions, kinetic and static coefficient of frictions (COFs) were found to be independent from the contact pressure. The static COFs were all found to be around 0.2, whereas the kinetic COFs were found to be higher in the low-met materials with respect to the NAO materials and they were found to be roughly independent from the contact pressure. The results were explained with reference to the adhesive theory of friction, highlighting the paramount role of the Fe-oxides embedded in the friction layer.

Sliding Behaviour of Friction Material Against Cermet Coatings: Pin-on-Disc Study of the Running-in Stage

Running-in is the initial stage during which a tribological system reaches a steady-state condition. In this study, the running-in behaviour of a commercial brake friction material, pin-on-disc tested under dry sliding conditions, has been investigated to understand the role of different surface finishing of hard coatings. These coatings were deposited onto cast iron counterface discs and then mechanically polished to achieve set values of average roughness, Ra, and roughness skewness, Rsk. The tribological data were modelled using an exponential relation for the wear rate, according to a literature approach. The model parameters were related to the above-mentioned disc surface roughness parameters. The results provided indications on the wear mechanisms acting during the sliding action. Furthermore, these mechanisms were correlated with the formation of the friction layers on the pins and on the disc wear tracks. Of particular relevance is the finding that a negatively skewed surface roughness is fundamental to achieve the best running-in performances. The beneficial effects, coming from this surface treatment, are derived from the reduction in abrasion and from the improved formation dynamics of the friction layer, with particular regard to those parts (secondary plateaus) made of compacted wear debris.

The Role of Counter-Face Roughness on the Tribological Performance of a Clutch System Tested with a Pin-On-Disc Tribometer

This study was a continuous investigation of the roles played by the tribofilm on dry automotive clutch system performance. Tribological experiments were performed by the addition of wear debris at the beginning of tribometer tests and by reducing the surface roughness of a cast iron counter-face. The initial surface conditions of cast discs were 0.2 and 1.2 µm. The pin-on-disc tests were carried out at three different PV levels: 3.08, 7.88, and 10.09 MPa·ms−1, and the current results were correlated to those previously obtained in the standard tribometer procedure. When the wear debris was added into the tribosystem, the friction coefficient level dropped drastically while the assembly wear rate rose. In contrast, the modified cast discs provided a reduced wear rate of assembly and a higher and more stable friction coefficient level. These improvements were obtained in a severe condition where higher temperature levels were reached. For the tests with added debris, SEM observations revealed a more intense tribofilm development over the worn surfaces of the clutch friction material. The smoothest cast disc did not damage the developed tribofilms and maintained them more stably due to a reduction in contact area stresses at the highest tribometer test.

Role of the friction layer in the high-temperature pin-on-disc study of a brake material

The tribological behavior of a commercial brake pad material, wear tested under dry sliding conditions against a cast iron counterface disc with a pin-on-disc apparatus, was investigated. Wear tests were conducted at the following disc temperatures: 25°C, 170°C, 200°C, 250°C, 300°C and 350°C. Above 170°C a transition from mild to severe wear was observed. At 25°C and 170°C, the friction layer on the pin surface consists of primary and compacted secondary plateaus. At 200°C and above, a progressive reduction of the pin surface coverage by the secondary plateaus, that are barely present after the 350°C test, is observed. Wear tracks on the discs derive from wear fragments due to the tribo-oxidation of the disc itself and from the wearing out of the pin material. The observed tribological behavior is very much influenced by the thermal degradation of the phenolic binder of the friction material, confirmed by thermogravimetric analyses, conducted on purpose on the pin material. Raman spectroscopy indicated the presence of carbonaceous products on the high temperature worn out pin surface. Although referring to rather extreme and simplified sliding conditions, the results obtained in this study provide useful indications on the role of the thermal stability of the organic component.

Improvement on Sliding Wear Behavior of Al/Cast Iron Tribopair by CNT's Reinforcement of an Al Alloy

The results presented in this work show the wear characterization of Al-Si matrix composites reinforced by multiwall carbon nanotubes (MWCNTs) under dry reciprocating sliding conditions against a grey cast iron (GCI) The wear resistance is investigated as a function of the carbon nanotube (CNT) content that varied from 2 to 6 wt%. The results demonstrated that the CNT content plays a relevant role in the wear behavior by substantially reducing the wear loss of Al-Si CNT composites. Further, it reduces the wear loss of the grey cast iron counterface. A physical model able to explain the improved behavior in both mating materials is depicted from experimental results.

Hybrid composites – Metallic and ceramic reinforcements influence on mechanical and wear behavior

This experimental study is concerned with the influence of metallic (Ti) and ceramic (SiC) reinforcements in an aluminumfsilicon (AlSi) alloy, when regarding tensile properties and wear behavior. Several micron sized particulate reinforced composites were produced by hot-pressing technique: AlSi–SiC and AlSi–Ti composites and AlSi-(Ti–SiC) hybrid composites.Regarding tensile properties, all composites presented higher ultimate tensile strength (UTS) than the AlSi matrix, with the highest UTS being attained by a hybrid composite (AlSi-11.25%Ti–5%SiC).Regarding wear behavior, reciprocating pin-on-plate wear tests were performed for unreinforced AlSi; AlSi–Ti composites and AlSi-(Ti–SiC) hybrid composite against a gray cast iron (GCI) counterface. The wear mechanisms for all the tested tribopairs are presented and discussed. It was observed that the wear behavior of the AlSi–Ti/GCI and also AlSi-(Ti–SiC)/GCI tribopairs are improved when compared with the AlSi/GCI system. AlSi-11.25%Ti-5%SiC hybrid composite exhibited the highest improvement in wear rate.

Interface analysis and wear behavior of Ni particulate reinforced aluminum–silicon composites produced by PM

This study is concerned with the influence of Nickel, as reinforcement, in an aluminum–silicon (AlSi) alloy when regarding wear behavior. For these composites, the effect of Ni content, in the tribopair performance, was evaluated. For this purpose, the pin but also the counterface wear behavior was analyzed.Nickel particulate reinforced aluminum–silicon (AlSi) composites, with 5, 12.5 and 20wt.% Ni were produced by a hot-pressing route. Microstructural characterization showed a uniform distribution of the Ni particulates in the AlSi matrix. EDS and XRD analyses revealed that the particle/matrix interface was formed by Al3Ni intermetallic. Reciprocating pin-on-plate wear tests were performed with AlSi and AlSi–Ni pins against a gray cast iron (GCI) counterface. It was observed that the wear behavior of the AlSi–Ni/GCI tribopair is improved when compared with the AlSi/GCI system.

The relationship between wear of semimetallic friction materials and pearlitic cast iron in dry sliding

Wear rate transitions for friction brake materials can be brought about by changes in the dominant wear mechanisms during operation. In order to investigate such effects, the relationship between the dry sliding wear rates of two commercial, semi-metallic friction materials, against a conventional cast iron counterface, have been investigated using a block-on-disc geometry. Applied load has been varied at a single, constant sliding velocity. Two distinct wear regions were recognized. The applied load for a transition in wear rate between these regions was 20% higher for one friction material than the other at contact temperatures in the range of 310–350°C. The specific wear rates of the semimetallic friction materials and of the cast iron counterface depended on the operating wear mechanism. In the first region, wear was mainly by abrasion and the wear rates are correlated in an almost linear manner. In the second region, wear was by two thermally-activated processes (thermal degradation of the phenolic resin for the friction materials; tribo-oxydation for the cast iron). The specific wear rates were correlated by a power-law relationship. In summary, wear was controlled by the abrasivity of friction materials in the first region and by the contact temperature in the second region.

Hot wear properties of ceramic and basalt fiber reinforced hybrid friction materials

In the present study, hybrid friction materials were manufactured using ceramic and basalt fibers. Ceramic fiber content was kept constant at 10 vol% and basalt fiber content was changed between 0 to 40 vol%. Mechanical properties and friction and wear characteristics of friction materials were determined using a pin-on-disc type apparatus against a cast iron counterface in the sliding speeds of 3.2–12.8 m/s, disc temperature of 100–350 °C and applied loads of 312.5–625 N. The worn surfaces of the specimens were examined by SEM. Experiments show that fiber content has a significant influence on the mechanical and tribological properties of the composites. The friction coefficient of the hybrid friction materials was increased with increasing additional basalt fiber content. But the specific wear rates of the composites decreased up to 30 vol% fiber content and then increased again above this value. The wear tests showed that the coefficient of friction decreases with increasing load and speed but increases with increasing disc temperature up to 300 °C. The most important factor effecting wear rate was the disc temperature followed by sliding speed. The materials showing higher specific wear rates gave relatively coarser wear particles. XRD studies showed that Fe and Fe 2O 3 were present in wear debris at severe wear conditions which is indicating the disc wear.

Influence of reinforcement volume fraction on sliding wear behaviour of SHS derived ferrous based metal matrix composite

Steel matrix composites reinforced with (W,Ti)C particles were processed by directaddition of an innovative powder to molten 0.4 wt-%Clow alloy steel.Thispowder was producedusing a self-propagatinghightemperature synthesis(SHS) reaction and consisted of a dispersion of fine (W,Ti)C particles (5-10 μm) in an iron binder. Dry unidirectional sliding wear behaviour ofthe compositematerialanditsunreinforced counterpartwas investigated atroomtemperature against a white cast iron counterface. In situ monitoring of wear and friction, in conjunction with electron microscopy, has been used tointerpretwearscar microstructures observed intermsofwear mechanisms. Wear experimentswere performedat a sliding speed of 1.5 m s-1 at different test loads. It was found that the wear resistance of the composite material was superior to that of their unreinforced counterparts over the entire range of loading employed during this investigation. The unreinforced base alloy exhibited a transition from mild to severe wear at a load of approximately 70 N. No such transition was found to occur for the composite materials. Instead, after a transient period of running in wear, steady state conditions were observed. At the highest level of carbide addition this transient period did not occur and the composite wore in a regime of mild wear over the full spectrum of loading utilised.

A comparison of the reciprocating sliding wear behaviour of steel based metal matrix composites processed from self-propagating high-temperature synthesised Fe–TiC and Fe–TiB 2 masteralloys

Steel matrix particulate composites were processed by direct addition of various powders to molten medium carbon steel. Fe–TiC and Fe–TiB 2 powders were produced using a self-propagating high-temperature synthesis (SHS) reaction and consisted of a dispersion of fine TiC (5–10 μm) and TiB 2 particles (2–5 μm), respectively in an iron binder. Addition of the Fe–TiC powder to the steel resulted in the formation of a metal matrix composite containing a homogeneous dispersion of TiC particles. However, addition of the Fe–TiB 2 powder resulted in the formation of a parasitic Fe 2B phase and TiC within the steel microstructure. In response to this an SHS masteralloy composed of Fe–(50% TiB 2+50%Ti) was manufactured which, when added to steel, prevented the formation of Fe 2B and resulted in a composite containing a mixture of TiB 2 and TiC particles. Dry reciprocating sliding wear behaviour of the three composite materials and their unreinforced counterpart was investigated at room temperature against a white cast iron counterface. Relative wear behaviour of the materials varied as a function of load. In all cases, the composite manufactured by addition of Fe–TiB 2 (yielding Fe 2B and TiC phases in the steel) exhibited wear rates greater than three times that of the unreinforced alloy. However, improvements in wear resistance over the base steel of up to two and a half times were observed with the other composites where the desired TiC and/or TiB 2 phases were retained in the steel. Scanning electron microscopy has been used to interpret wear behaviour in relation to both the as-cast microstructures of the composites and the wear scar microstructures observed.

Elevated temperature sliding wear behaviour of TiC-reinforced steel matrix composites

TiC-steel matrix particulate composites were processed by direct addition of powder to molten BS970:070M15 steel. This powder was produced using a self-propagating high-temperature synthesis (SHS) reaction and consisted of a dispersion of fine TiC particles (5–10 μm) in an iron binder. Dry reciprocating sliding wear behaviour of the composite material and its unreinforced counterpart was investigated at room temperature, 250 and 500°C against a white cast iron counterface. In situ monitoring of wear, in conjunction with electron microscopy, has been used to interpret wear behaviour in relation to the wear scar microstructures observed. Wear experiments were performed at an average sliding speed of 0.1 m s −1 at different test loads. At room temperature, both materials exhibited a marked increase in wear rate at a particular load, indicative of a transition to a more severe regime of wear. This occurred at a significantly higher load for the TiC-reinforced composite than for the unreinforced steel. Prior to the onset of this transition, the reinforced steel displayed significantly superior wear resistance. Above this transition load, however, the reinforced material exhibited similar wear rates to those of its unreinforced counterpart. At a test temperature of 250°C, the transition to the more severe wear regime was observed to occur at a substantially higher load for the TiC-reinforced steel than for the unreinforced material. At 500°C, wear at the interface is dominated by oxidation and both materials exhibited mild wear (characterised by extremely low wear rates) over the range of loads considered in these experiments.

Tribological properties of centrifugally cast copper alloy-graphite particle composite

Centrifugal casting technique was used to impart better tribological properties to the inner periphery of centrifugal castings of a C90300 copper alloy originally containing 13 vol pct graphite particles. Microstructural observation of centrifugally cast copper alloy containing graphite particles shows that a graphite-free zone and a graphite-rich zone (25 vol pct) with a unique microstructure are formed near the outer and the inner periphery of the centrifugally cast cylinders, respectively. Wear tests were conducted using a pin-on-disc apparatus running against a cast-iron counterface under dry conditions at applied loads between 44.5 and 267 N and at a sliding speed of 1 m/s. The worn surfaces of pin and counterface were analyzed using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. The wear rate (1.89×10−13 to 7.59 × 10−13 m3/m) and the temperature (50 °C to 170 °C) at the counterface for the pins from the graphite-rich zone of the centrifugal castings were found to be lower than the friction coefficient (0.52 to 0.75), the wear rate (6.32×10−12 to 3.16 × 10−11 m3/m), and the temperature (70 °C to 200 °C) at the counterface for the pins from the graphite-free zone (of the same centrifugal casting under similar conditions). A greater transfer of the copper phases from the pin to the cast-iron counterface was observed visually from the pin of the graphite-free zone than from the pin of the graphite-rich zone, which was also confirmed by EDX analysis. This leads to an increase in the weight of the counterface running against the pin from the graphite-free zone with an increase in the applied load. Despite the presence of graphite in cast iron, the presence of graphite in the matrix of mating copper alloys lead to improved tribological properties. The effect of graphite particles on tribological properties of the composites was discussed in terms of the transfer of iron and copper phases, the interparticle distance between graphite in cast-iron and copper-graphite alloys, and the deformability of the matrix containing graphite.

The effect of humidity on friction and wear of an aluminium–silicon eutectic alloy

The sliding wear of an aluminium–silicon eutectic alloy against cast iron counterface in 3–100% relative humidity range has been investigated. The results show that the moisture content has a significant effect on the friction and wear of the Al–Si alloy. The wear rate decreases by two orders of magnitude as the relative humidity increases from 3% to 100%. At low humidity conditions adhesive wear is predominant, whilst at high humidity conditions a layer of compacted oxide–metal debris film is formed on the Al–Si slider surface, which reduces the direct metal–metal contact. The friction coefficient is maximum at 3% and 100% relative humidity conditions.

Microstructural changes induced by dry sliding wear of a A357/SiC metal matrix composite

The sliding wear behaviour of a commercial (Duralcan) metal matrix composite containing 30 vol.-%SiC particles, and that of the unreinforced A357 matrix alloy were investigated in the load range 6-74 N and a sliding speed of 1 m s−1 against a grey castiron counterface. At the lowest load (6 N) the wear coefficient of the composite (3.1 × 10−6 mm3 N−1 m−1) was lower than that of the monolithic alloy (169.6 × 10−6 mm3 N−1 m−1 ). Between 18 and 40 N the wear performance of the two materials was verysimilar but at the highest load (74 N) the wear coefficient of the composite (176.8 × 10−6 mm3 N−1 m−1) was much higher thanthat of the monolithic alloy (89 × 10−6 mm3 N−1 m−1). The maximum depth offriction induced deformation wasfound to beabout three times higher for the composite than the monolithic alloy at a load of 74 N. Detailed TEM was performed on the surface of the composite. The SiC was found to be fragmented. Extensive formation of a alumina was observed, and the presenceof iron oxides indicated transfer from the counterface. Widespread cracking was found at the SiC/matrix and lI. alumina/matrixinterfaces. The matrix had developed a fine subgrain structure with little evidence of the Mg2Si which was present in the startingstructure.

Microstructural changes induced by dry sliding wear of a A357/SiC metal matrix composite

The sliding wear behaviour of a commercial (Duralcan) metal matrix composite containing 30 vol.-%SiC particles, and that of the unreinforced A357 matrix alloy were investigated in the load range 6-74 N and a sliding speed of 1 m s−1 against a grey castiron counterface. At the lowest load (6 N) the wear coefficient of the composite (3.1 × 10−6 mm3 N−1 m−1) was lower than that of the monolithic alloy (169.6 × 10−6 mm3 N−1 m−1 ). Between 18 and 40 N the wear performance of the two materials was verysimilar but at the highest load (74 N) the wear coefficient of the composite (176.8 × 10−6 mm3 N−1 m−1) was much higher thanthat of the monolithic alloy (89 × 10−6 mm3 N−1 m−1). The maximum depth offriction induced deformation wasfound to beabout three times higher for the composite than the monolithic alloy at a load of 74 N. Detailed TEM was performed on the surface of the composite. The SiC was found to be fragmented. Extensive formation of a alumina was observed, and the presenceof iron oxides indicated transfer from the counterface. Widespread cracking was found at the SiC/matrix and lI. alumina/matrixinterfaces. The matrix had developed a fine subgrain structure with little evidence of the Mg2Si which was present in the startingstructure.

Effect of humidity on friction and wear of AlSi eutectic alloy and AlSi alloy-graphite composites

The influence of moisture content in air on the unlubricated sliding wear of an AlSi eutectic alloy and an AlSi alloy-graphite composite material containing 55 vol% graphite particles has been investigated. Experiments were conducted in a relative humidity (RH) range of 1–95% using a cast iron counterface. In the humidity range of 1–70% RH the mild wear of the AlSi alloy was characterized by the formation of an iron-rich compacted surface layer. On increasing the humidity content from 70% to 95%, the wear rate decreased by two orders of magnitude owing to the in situ formation of an oxide-rich surface film. The tribological behavior of the AlSi alloy-graphite composite material closely resembled that of graphite. In ambient air its coefficient of friction was low (0.2-0.25) and its wear rate was three orders of magnitude lower than that of the base alloy. However, an abnormally high friction and wear behavior was observed for the composite material at 1% RH.