The results showed that the adhesion, friction coefficient and wear rate of the piston ring with ductile iron base material is better than that of gray cast iron. Compared to gray cast iron, the adhesion of ductile iron to the coating increased by 66%. The piston ring wear rate of ductile iron was reduced by up to 36% at 40 N of normal force compared to gray cast iron.
The results of this work showed the promising applications of chromium-based diamond coating on ductile iron or piston ring in field conditions.
INTRODUCTION
Background and motivation
Ductile iron is defined by the presence of graphite in the form of spherical nodules. The types of failure observed in the scratch test depend to a large extent on the properties of both the substrate and the coating. In this case, significant plastic deformation will occur internally and the critical load of the scratch test can be defined as the load at which the coating is scraped off, exposing the substrate.
This usually limits the scratch test to the assessment of hard coatings on softer substrates.
Objective of thesis
Organization of thesis
MATERIALS AND METHODS
Materials
The values determined in this work agree with the hardness of the Cr-diamond coating from the literature [10, 11]. The modulus of elasticity of the Cr-diamond coating was determined by the nanoindentation method on the surface of the coating with a Berkovich tip. 3(f) shows an example of a load-displacement curve derived from a nano-indentation test of two specimens having a similar shape.
It was found that the elastic modulus of Cr diamond coating on samples is similar. It was found that the porosity of the coating of the two samples was comparable. Figure 3(h) and (i) show the image of the Cr-diamond coating surface of samples A and B along with their cross-sectional profiles using a confocal laser microscope.
3(j) shows the three-dimensional image of the cross-pattern area of the Cr-diamond coating of specimens A and B. The arithmetic mean surface roughness of this area of the Cr-diamond coating of specimens A and B obtained by confocal laser scanning microscopy data at three locations with five specimens at µm × µm scan size were determined to be approximately µm and µm (mean ± 1 standard deviation), respectively. The results show that the surface roughness of the cross-pattern area of the Cr diamond coating of the A specimens is slightly higher, about 15 %, than the B specimen.
Cross-sectional images of samples (a) A and (b) B, SEM images of interface of samples (c) A and (d) B, (e) hardness, (f) load-displacement curve of coating, (g) porosity of coating, surface images of samples (h) A and (i) B corresponding to cross-section profile, (j) three-dimensional images of samples A and B. Cross-section images of samples (a) A and (b) B, SEM images of interface of samples (c) A and (d) B, (e) hardness, (f) load-displacement curve of coating, (g) porosity of coating, images of surface of samples (h) A and (i) B correspond to cross-sectional profile, (j) three-dimensional images of samples A and B.
Methods
- Scratch adhesion test
- Pin-on-reciprocating test
Therefore, after polishing, the average coating layer thickness was determined at randomly selected five different locations on the cross-section of five samples. 4(a) shows the photograph of the indenter along with an example of three-dimensional laser scanning microscope images. At the point of failure, the depth of the scratch mark is equal to the average thickness of the coating layer.
The friction and wear behavior of a Cr-diamond coated piston ring under oil lubrication was investigated using a pin tribotester. The average bead radius was determined from the bead surface profile obtained from laser scanning confocal microscopy data, per mm (mean ± 1 standard deviation). 4(b) shows a photograph of a ball glued to a holder together with an example of three-dimensional laser scanning confocal microscopy images.
A photograph of the experimental setup of the pin tester in this work is shown in Fig. Average cross-sectional height profiles were recorded in all areas of the wear track. Wear volume was calculated by multiplying the cross-sectional area of the wear track and the length of the wear track.
The wear rate of the plate was calculated by the wear volume divided by normal force multiplied by the sliding distance. Photograph of (a) scratch adhesion tester, scratch sample, indentation finger and three-dimensional image of indentation, (b) pin reciprocating tribo-tester, plate sample, pin sample and three-dimensional of the Cr steel ball.
RESULTS AND DISCUSSION
Adhesion characteristics
Figure 5(c) summarizes the wear failure modes during the etching process of sample B shown in Fig. The failure modes in two specimens were also observed in the optical images, which is spalling from the brittle fracture of the coating material separating in front of the moving indentation. As the force increases, spalling leads to cracking at the edge of the scratch track, as shown in the optical images of both specimens, and the failure mode became stress spalling.
Due to the great pressure for the indenter, when the indenter moves down, it causes the top coating to break and splash on the sides of the scratch track, which may be the plowing effect for the moving indenter. The same failure mode is also shown in Mouche's study for Cr coated on SiC [21]. The average adhesion work of the coating layers of samples A and B after calculation was shown in Fig.
Hainsworth's research for TiN coating also showed the effect of plastic deformation of the substrate layer on scratch performance [22]. Comparison with the results of other ceramic coatings, a very thin Cr plate coated on 25Cr3Mo3NiNb barrel steel substrate using electroplating method, which has the interfacial fracture energy in the range of 756 J.m-2 to 1514 J.m-2 [23]. The initial thickness of the coating layer is much greater than the threshold thickness in the Bull and Rickerby model.
The boundary between the coating layer and the substrate is curved, resulting in uneven thickness after polishing. Therefore, future studies should be performed with samples whose coating layer-substrate interface is flat and the thickness of the coating layer is within the limitations of the model.
Tribology characteristics
Figure 7(a) shows optical images corresponding to cross-sectional height profiles of the Cr steel balls before and after the experiments. 7(b) and (c), the increase of normal force leads to the reduction of the wear volume and wear rate of the ball. The wear volume and wear rate at 10 N normal force is higher than that of other normal forces.
In the experiment with sample B performed under 10 N, the wear volume was not clearly measurable, and the width of the wear scar was measured to be 306.9 µm. It was observed that the width of the wear tracks on two samples did not differ much with the increasing normal force. Under the normal force of 10 N, the wear volume of the samples is negligible, so the wear rate is also very low compared to other normal forces.
The reason may be that the wear rate is proportional to the wear volume and inversely proportional to the normal force. The normal force also increases and the amount of wear increases, so the wear rate will not be much different. It also agrees with the trend of sample wear volume in this work.
In general, the results show that the wear of sample B is slightly better than that of sample A. a) Optical images corresponding to the cross-sectional profile of wear marks formed on the plates with different normal force, (b) wear volume, (c ) plate wear rate relative to normal force. Figure 9(a) shows an image of the wear track formed on the sample B under the normal force of 30 N. The average surface roughness at five different locations with scan size 100 µm × 100 µm within the wear track in Fig.
The effect of the different base materials on the tribological properties of the coating is not clear because the thickness of the coating is much larger than the height of the wear track.
CONCLUSIONS
The results in terms of friction and wear are not affected by the difference between the mechanical properties of coatings, because the results of hardness and modulus of elasticity of coatings have been measured on two grades of cast iron, which show similarities. The reason may be that the surface roughness of the Cr diamond coating on ductile iron has been adjusted to be 15% smaller than gray cast iron. The best wear rate reduction of Cr diamond coating on nodular cast iron compared to gray cast iron was 36% at a normal force of 40 N.
The result of this work can contribute to a better understanding of the effect of surface treatment on the friction reduction and wear resistance of Cr-diamond coating, thereby providing a fundamental basis for the design of the coating to extend the service life of the piston ring. Ficici, "Studies of Friction and Wear Between a Cylinder Liner and a Pair of Piston Rings Using the Taguchi Design Method," Advances in Engineering Software, vol. Xie, “Effects of surface texturing on the tribological behavior of piston rings under lubricated conditions,” Industrial Lubrication and Tribology , vol.
Shah, "The influence of a piston ring coating on the wear and friction generated during linear oscillation", Lubricants, vol. Dahm, “Wear Response of Crystalline Nanocomposite and Glassy Al 2 O 3 SiC Coatings Subjected to Simulated Piston Ring/Cylinder Wall. Zhang, “Comparison of the Abrasion Behavior and Wear Resistance of Candidate Engineered Coatings for Automotive Piston Rings,” Tribology International, vol.
Xiao, “Measurement of fracture toughness and ultimate shear strength of hard and brittle Cr coating on ductile steel substrate,” Surface Engineering , vol. Berasetegui, "Chapter 7 - A Review of the Potential of Quantitative Measurement of Coating Adhesion Using Scratch Testing," in Tribology and Interface Engineering Series.
