Ceramic Particle Reinforced Camshaft Lobes: A Performance Evaluation and Comparative Analysis

the


INTRODUCTION (GİRİŞ)
The fuel injection during the engine operation into the combustion chamber is controlled by the cams that are attached to the camshafts.Therefore, the physical and operational characteristics of the cams can significantly influence the operational capabilities of the engine.High wear resistance and a long lifespan are necessary for integration as camshaft lobes.The successful integration of ceramic-reinforced lightweight cams into camshafts can lead to low production costs and fuel consumption, as ceramic-based composites possess excellent wear resistance and lightweight characteristics.
The failures in automotive vehicles are divided into isolated and widespread failures that are occurred due to abuse, age, human-machine interaction errors, and improper maintenance.Heyes et al. [1] studied 70 cases of vehicle component failures and reported a striking 41% share by the engine components.By using appropriate material selection, design, and processing techniques during the manufacturing stage and by properly evaluating the performance of different engine components, failures can be prevented.
The feasibility of using the Al matrix ceramic reinforced cams as a replacement to conventionally used high alloy steels [2][3][4] was investigated in the previous study [5].However, results showed that the behavior of the cams was influenced not only by the types and ratios of the ceramic (B4C or SiC) particles, but also by the size of the reinforcement particles.However, since the mentioned study [5] was intended to develop a virtual engine-like wear environment and conduct the feasibility analysis of the ceramic reinforced Al matrix composite as cams, influence of longer durations of the wear tests was not reported.But for a proper representation of said composites to be claimed as a replacement to conventional steels cams, performance evaluation is inevitable.
Barothi et al. [6] attempted to improve the engine performance by manipulating the camshaft lobe geometry.Results showed that changing the lobe lift height from 12 mm to 12.43 mm resulted in a 4.1% increase in cylinder filling efficiency during intake stroke.However, the power consumption was slightly increased (from 323.72 kW to 334.18 kW) owing to modified lift height in the lobes.Burdzik et al. [7] collected 40 cams from Cinquecento 700 engine that had already been exposed to real-life wear for a range of 1250 to 48500 km.It was reported that while wear intensity was fairly dependent on the cam angles, the justifications to these maxima required incorporation of pressure calculations.Overall, the wear behavior was independent of the type of cams used for assessment.Godino et al. [8] analyzed the recurring major failure in the valve train system (engine valves opening/closing system) of 130 buses (out of 400 urban buses fleet) and reported excessive damage on the cam nose, especially after 500 thousand km.The reasons for the failures, after a systematic analysis, were declared to be a combination of wear (low hardness of the cam nose) and fatigue cracking in the cam subsurface.
Using ceramic particles in a matrix with improved interface compatibility is an obvious way to counteract wear in mechanical parts that interact physically; ceramic particle reinforced Al matrix composites are a success from an engineering standpoint.However, as the sliding distance (or duration of wear exposure) increases, the wear response might not be similar throughout the experiment [11].Supposing laterally uniform microstructure, friction induced elevated temperatures, fatigue induced surficial changes, and most importantly, the deformation induced hardening of the top surface triggers microstructural evolution that is absent at the initial stages of the wearing [16].Such a situation necessitates considering the extended duration wear tests while defining absolute wear behavior of the composites.Shinde et al. [11] manufactured Al-12Si matrix B4C using ultrasonic stir casting and studied the influence of speed and sliding distance on the microstructure during wear.While the friction of coefficient remained fairly unchanged, the wear behavior was complex with respect to the sliding distancecombined influence of additional factors of load and sliding speed needed to be incorporated as well.Muley et al. [17] reported that ceramic reinforced metallic composites (Mg alloy-AZ31B + 1.5wt% Al2O3-1wt% Ca [9], Al 2219/SiCp + Grp [18], and Al alloy LM25 + SiCp + Gr [16]) show different wear behavior at different stages of the sliding distance.For example, Alidokht et al. [9] reported that wear mode changed from ploughing caused by ceramic particles to abrasion and adhesion after 1500 m mark for Mg+Al2O3.Basavarajappa et al. [18] also reported a non-linear increase in the wear rate with increasing sliding distances.The wear mode changed again after 1500 m mark to abrasive wear that was caused by the cracked Si3N4 particles.Raju et al. [12] examined the wear of SiC and AlN hybrid composites for up to 1000 m and reported improved wear resistance compared to single-particle reinforced composites and attributed it to synergistic effects at the interface.It can be seen from Table 1 that although studies on the medium sliding distance of the ceramic particle reinforced Al matrix composites at the laboratory scale are available, extended sliding distance/durations are not as abundant.A contributing factor to this is the smaller size of the specimens and the constraints of the tribologyrelated machinery.In order to solve this problem, a much-automated system with minimum performance degradation must be prepared.The working principles and the component details of the setup are already detailed in the previous study [5].The setup mimics the real-life engine conditions based on the principles of the valve train system (actuation of engine valves using lobes on the camshaft) in dry and wet conditions.Setup has monitoring capabilities such as checking the temperature of the oil and angular speed wheel.

Numuneleri
Starting materials were based on the powder forms of SiC (two particle size), B4C (single particle size) and Al AA2124 alloy.AA2124 aluminum alloy is known for exceptional strength, fatigue resistance, and adequate weldability and is widely used in the aircraft and automotive industry as critical weightto-strength sensitive structural components such as wing skins and pistons.B4C and SiC are famous for having exceptional hardness and thermal resilience making them appropriate candidates for various fields.For example, B4C is industrially known as an advanced ceramic used against ballistic threats, while SiC is used in the furnaces and resists oxidation for inherent chemical inertness.
Only the reinforcement ratios of 20 and 30 vol. % were used in the current study because the wear behavior of the ceramic particles containing 10 vol.% did not differ significantly.Moreover, the reinforcement particles, which were smaller in size (B4C: 1-7 µm, SiC: 2 µm), showed a better wear performance compared to larger ceramic particle size (SiC: 20 µm) composites [5].For these reasons, only the 20 and 30 vol% SiC with an average particle size of 2 µm were used in the studythe selected raw materials are shown in Figure 1(a).
Samples are prepared using powder metallurgy in three steps: mixture preparation, powder mixing, and hot pressing in a mold.The parameters are detailed in Figure 1(b).Test samples are prepared with an increasing fraction (20 and 30 vol.% SiC) of ceramic particles (Figure 1(c)).These samples were selected after the feasibility tests conducted in the previous study [5] on a broader range of types of composites (Figure 1(c)).The final product is in the camshaft lobe form because of the geometry of the specially prepared mold (Figure 1(d)).The detailed images of the mold components can be found in the previous paper [5].Finally, reference materials for validating and comparing the wear behavior were also prepared, as shown in Figure 1(e).
One of the major issues related to the manufacturing of powder metallurgy composites is the presence of porosity.While the reduction in the porosity is related to the powder particles' characteristics (size, size distribution, morphology), pressure (compaction during cold/hot pressing for increasing inter-particle contacts [19]), temperature (for initiating the mass transport at interparticle boundaries) and finally the duration of the treatment (in case chemical reactions happen at the interfaces that depend on atomic diffusion [20]), the pressure and temperature are most influential.Therefore, in order to achieve the highest density in the composites, different pressure-temperature values were attempted before the cams were produced.
As mentioned above, 200, 400, 600, and 800 MPa were attempted at temperatures of 23, 250, 450, and 650 °C for the samples reinforced with 10, 20, and 30 vol.% SiC.The maximum densities these composites reached (taking 100% as the theoretical density) are shown in Figure 2. It can be seen that at lower temperatures, it requires higher pressure values in order to reach closer to the theoretical density as shown in Figure 2(a).Additionally, the sintering of samples in which ceramic particles are present is unsatisfactory as compared to the nonreinforced samples.This is attributed to the ceramic nature and negligible yielding of the ceramic particles [21] under the pressure (Figure 2(a)).Regardless of the temperatures used, higher compression values increase the density of the composites that reaches a maximum value at 650 °C for 800 MPa as shown in (Figure 2(d)).This was the reason all composites were sintered at these parameters for 30 minutes.
Other than the composites used in the study, reference nodular graphite cast iron (NGCI) was also subjected to several characterizing stages in order to see the mechanical aspects of the used materials.That is to say that while the hardness of each specimen was measured individually, the hardness of the NGCI was also measured and it was checked whether it required any further hardening practices.For example, in literature authors have reported increasing the mechanical properties of the camshaft lobes using several treatments (chills to acquire ledeburitic surface [3], induction hardening [22], austempring of the ductile irons [23], and cathodic electrolytic plasma hardening [24]).Figure 3 shows NGCI has some influence of the induction hardening (8%) but almost negligible influence of the post hardening griding operation (0.1%).As for the composites, the hardness increased with the increasing content of the ceramic particles regardless of the types of ceramics used.
Finally, the images of the manufactured selected cams before and after the wear tests are given in Figure 4(a).Later, samples were perforated after being removed from the mold to fit the shaft as shown in Figure 4(b).Additionally, the bearings, that were used as the counterface, are also shown in Figure 4(b).

Wear Performance Tests (Aşınma Peformans
Testleri) Wear tests were conducted using a setup which is explained in detail somewhere else [5].For reference, the tests were conducted under wet conditions in a virtual engine-like environment.Regardless of the working principles of the specially designed virtual engine-like environment for the wear tests, the load on the samples (70, 90, 115, and 140 N), test durations (2.5, 5, 7.5, and 15 h), and angular speed (500 rpm) could be controlled.The wear tests were conducted using a commercial oil (60 mlt DEW® SAE10W40) that could be heated (and maintained) to a temperature of 90 °C.The density of each sample was measured using the Archimedes principle.The mass of the samples before and after the wear tests was measured.Using the change in the mass (∆m) and the density (ρ), the volume of the material loss was calculated.Later, this volume was divided by the product of time and contact load to calculate the wear rate (W).The wear resistance is calculated by taking the inverse of the wear rate and it is further divided by the density of the sample to measure the specific wear resistance of the sample.

RESULTS (BULGULAR)
It was reported in the previous study [5] that overall, the wear rate of the composites decreased with the increase in the ceramic content under dry wear.Only the results from the 2 µm reinforced specimens are given for the performance evaluation tests.There are two reasons behind that: i) the wear rate of the composites which had larger ceramic reinforcements (20 µm compared to 2 µm) initiated three body abrasive wear (especially A30S_20 specimens) and reduced the wear resistance of composites.ii) the difference in the wear rate was more prominent for the 20 and 30 vol. % ceramic content.In other words, based on the results of the previous study, only the best performing samples are selected for the extended duration wear performance evaluation.Finally, the lobes made of merely the Al matrix could not complete the testing because it was pasted to the counterface.With these realizations presented, performance wear test results are given in Figure 5.
It can be seen from the Figure 5 that the wear rate of the composites is slightly higher than the cast iron, especially at shorter durations.However, as the duration of the wear increases, the overall wear rate is almost similar for all the samples.Additionally, the higher ceramic content reduces the wear in the composites, much significantly, at the lower sliding distances/war durations.Reduced wear in the composites has been reported by various authors such as Bhowmik et al. [25] and Uzkut et al. [26] with increasing SiC content.
One of the important issues related to the wear of the metals or metal matrix composites is the plastic deformation due to repeating action of sliding.For example, when two surfaces interact for a longer duration, the top surface is plastically deformed and hardened [27].Since samples are also subjected to a force of 115 N, the phenomenon of deformation under loading is further encouraged.The wearing of such a hardened surface can happen under two circumstances: a) spalling [10] that is possible if there are major cracks or grooves formed during wear in the subsurface, and b) further increasing the load so that cracks could be formed in the hardened surface.In comparison to the fresh surface, the hardened surface that has deformed past its plastic limit is more difficult to wear in both situations.Because of this, regardless of the ratios of reinforcement, composites exhibit somewhat less wear over extended periods of time, as shown in Figure 5(a).
The A30S_2 and NGCI specimens exhibit comparable wear loss in the composites, but their wear behaviors are not the same.It should be mentioned that the study's goal is to offer a lightweight substitute for the cast iron camshaft lobes that are currently in use.The benefits of adding ceramic-reinforced camshaft lobes are thus much more accurately represented when the wear resistance (inverse of wear rate) is divided by the density of the tested samples.The specific wear resistance of the camshaft lobes tested in this study is given in Figure 6 and the advantage of the A30S_2 specimen can be clearly seen.
It's critical to realize that a mechanical component's wear cannot be addressed separately.It is imperative that a component not be added to the machine if it exhibits adequate wear resistance but results in excessive wear on the counterface.Therefore, current study reports the results of the wear tests in conjunction with the wear in the counterface.Figure 7 shows the combined wear resistance of the counterface and the composite lobes.
It can be seen in Figure 7(a) that although wear resistance of the 30 vol.% SiC reinforced composite lobes is higher; it occurs on the expense of excessive wear in the counterfacea situation that is not preferred.Although the wear resistance of the 20 vol.% SiC composites is lower, it shows a fairly (almost 1:1) ratio of the wear that is divided between counterface and composite lobe.Figure 7(b) shows the comparative specific wear resistance of the samples taking the wear resistance of the NGCI as 100%.Here again, the specific wear resistance of the A20S_2 is lower than the A30S_2.
In order to analyze the influence of the impact force on the wear resistance of the composites, some samples were selected to complete the wear tests at increasing loads.The load profiles for increasing loads at the tip (180° represents tip) are shown in Hata! Başvuru kaynağı bulunamadı.(a).The d etails about the springs compression and the methodology that is used for calculating the load value is given in detail somewhere else [5].One of the influences of the higher loads is the reduction in the vibration between the contacting surfaces of counterface and camshaft lobe.Reduction in the vibration also reduces the repeating impacts on the wear specimen.As the angular speeds increase, so does the impact intensity.Because of this, the composites' wear resistance is increased, and their wear rate is slightly reduced as a result of the higher load values as shown in Hata! Başvuru kaynağı b ulunamadı.(b).Alidokht et al. [9] reported a slight increase in the wear loss 25% for increasing load from 10 to 25 N and 30% for increasing load from 25 N to 40 N for Al + SiC composites.Other authors [12][13][14][15] have also reported the influencing of increasing loads on the wear behavior of similar composites (see Table 1), however, the results might not agree with the current experiment.The reason behind that is the difference in the wear setup mechanism (such as reduction of vibration at increasing loads) peculiar to the current wear evaluation setup this project uses.As for the wear of the counterface, except for the NGCI specimen, the wear rate remained fairly similar at all wear durations (Figure 5(b, c, d)).The reason for higher wear in the counterface against the NGCI is the higher hardness [28] of NGCI which in turn was intentionally improved using the induction hardness treatment [22] (Figure 3) at shorter durations.As the wear duration increases, the surface of the counterface or the NGCI is deformation-strengthened and wear in both interacting faces is reduced.Additionally, Figure 7(c) shows that the worn surface of the NGCI lobes has sliding wear traces where aggressive grooves are not formed.The counterface, while being worn against NGCI, also lacks excessive wear traces As for the closing ramp, the roughness is slightly lower than the tip since this region of the cam does not counter the load imposed by the springs.Our previous study [5] as well as Burdzik et al. [7] reported a region dependency of wear in the camshaft lobes after 2.5 h and a distance of 1250 -48500 km, respectively.
The microstructure of the counterface reveals that the wear scratches on the counterface are not considerable deep (Hata!Başvuru kaynağı b ulunamadı.(g))enough to increase the surface roughness dramatically (Hata!Başvuru kaynağı b ulunamadı.(e)).This behavior is attributed to the smaller size (2 µm) of the ceramic particles.However, it must be noted that the there was a significant wear in the counterface against the A30S_2 specimen (as shown in Figure 7

CONCLUSIONS (SONUÇLAR)
This study reported the performance potential of ceramic-reinforced Al matrix composites (CMMCs) as lightweight alternatives to cast iron camshafts in engines, with a particular focus on their wear performance under semi-engine like conditions.Following conclusions from the study can be deduced.
• Ceramic reinforced Al matrix composites exhibited a positive correlation in wear with higher SiC content (20% and 30% tested), showcasing their potential for weight reduction without significantly compromising the performance.
• As the duration of the experiment increased, the wear loss of the composites showed similar values due to formation of hardened top surface.The surface is hardened due to fatigue and sliding under loads that yields the metallic matrix beyond its elastic limit.
• Al + 20 vol.% SiC composite showed adequate wear resistance with a little counterface wear, offering a practical balance between durability and component protection.As for the Al + 30 % SiC, it showed superior wear resistance, but it triggered significant counterface wear due to dislodged SiC particles that initiated three-body abrasive wear.

Figure A :
Figure A: Manufacturing, wear performance and characterization of wear behavior of particle reinforced Al matrix cams./ Şekil A: Parçacık takviyeli Al matrisli kamların üretimi, aşınma performansı ve karakterizasyonu.

Figure 7 (Figure 8 .Figure 9 .Figure 10 .
Figure 8. a) The load profiles of the camshaft lobes and b) influence of increasing applied loads on the specific wear resistance of the camshaft lobes (a) yük profilleri ve b) artan yüklerin kamların spesifik aşınma direnci üzerindeki etkisi) (a)).So, if the surface roughness of the counterface is lower, wear has to occur via mechanism of excessive ceramic particles that must have initiated the threebody abrasive wear and not via the mechanism of the aggressive ploughing in the counterface.This claim is supported by the transferring and embedding of the SiC particles from the composite to the counterface as shown in Hata! Başvuru k aynağı bulunamadı.(f) and confirmed with the EDS analysis shown in Hata! Başvuru kaynağı b ulunamadı.(h).Because of the viscous oil, some of the counterface's broken particles remain inside the wear system for an extended period of time, as shown in Figure7.

Figure 10
schematically shows the initiation of three body abrasive wear due to dislodged ceramic particles (annotation A) and formation of hardened layer due to repeating sliding loads (annotation C) for a longer (above 10 hours) duration.

Table 1 .
Selected studies from the literature dealing with short/medium/long distance laboratory research and case studies (Literatürden kısa/orta/uzun mesafeli laboratuvar araştırmaları ve vaka incelemeleri ile ilgili seçilmiş *: Estimation, nr: Not Reported, D: diameter of specimen, l: length of specimen, t: thickness of specimen, h: width of specimen, rpm: rounds of disk per minute, HRC: Rockwell C hardness,Current study deals with the extended duration (2.5, 5, 7.5, 15 h) wear of the camshaft lobes of Al+SiC and Al+B4C composites under dry and wet conditions.While the composites are manufactured by a combination of stir casting and squeeze casting in a mold of lobe geometry, results were compared with reference samples (AA2124 matrix and nodular graphite cast iron).