The Wear Resistance of Cryogenically Treated Diamond-Like Carbon Films

This study explored the effects of cryogenic treatment on the microstructure, hardness, and wear-resistance of diamond-like carbon (DLC) by cryogenically treating NAK 80 mold steel coated with DLC. Raman spectroscopy analyzed the structure of the DLC film. Nanoindenter analyzed the hardness and Young’s modulus of the film, and their relationship determined the wear resistance. Wear test assessed the wear rate and friction coefficient of the DLC film. The results showed that cryogenic treatment increased the rate of carbide precipitation and refined the grain structure. Raman spectroscopy indicated that the Raman intensity rate (ID/IG) of treated DLC films was smaller than those without cryogenic treatment. When the sp3 bond increased, the hardness and wearresistance of the DLC film also increased.


Introduction
Cold treatments primarily involve subzero treatment and cryogenic treatment.Ultra cryogenic treatment performed in this study is also known as ultra-cold treatment and refers to a method of processing materials at temperatures below -196 ºC and is an extension of cryogenic treatment.Cryogenic treatment not only substantially improves the mechanical properties and lifetime of ferrous metals, non-ferrous alloys, carbides, plastics (including nylon and Teflon), silicates, and other materials, but also stabilizes material size, improves uniformity, reduces deformation, exerts no damage on work pieces, generates no pollution, and has considerable economic benefits and market prospects.The principle involves using both liquid oxygen and liquid nitrogen as cooling materials to reduce the temperature below 0 ºC.[3][4] The structure of the diamond-like carbon (DLC) film is a mixture of sp 2 graphite plane bonds and sp 3 tetrahedron diamond covalent bonds.With a high proportion of sp 2 /sp 3 , its structure and properties are similar to that of a natural diamond.Specifically, the structure is similar to a diamond because it possesses the properties of high chemical stability, a low friction coefficient, excellent thermal conductivity, low adhesion, high wear resistance, and high hardness (HV 1500-5000). 5Experiment

Experimental procedure
In this experiment, NAK80 mold steel specimens were employed.The composition of these specimens is listed in Table 1.The experiment was divided into three stages.In the first stage, the cathodic arc plasma coating method was applied under a fixed target voltage, and a graphite target was used as the DLC film source.Its coating parameters are presented in Tables 2 and 3.
In the second stage, the hydrogen DLC film and the substrates underwent cryogenic treatment before slowly cooling to the temperature of liquid work pieces or substrates.Tables 2 and 3    were also obtained.

Wear resistance test
In this experiment, a pin-on-disk wear testing machine was used to measure the friction coefficient and wear-resistance characteristics of the coating.
Tungsten carbide balls were used to compare the specimens and level of wear to determine the friction coefficient (μ) and wear rates.The wear rates were then used to assess the wear-resistance characteristics.
During the experiment, the relative humidity was 53%, and the temperature was 22 ºC.The parameters    5 and Fig. 4. As indicated by the data provided in Table 5, when a minimal amount of silicon was added, the Raman-analyzed G-band shifted slightly toward lower

Table 5 Raman intensity ratio (ID/IG) analysis
wavenumbers.An increase in the half width and height of the G-band and a decline in the Raman intensity ratio (I D /I G ) were observed.In addition, the graphite-like pieces in the film shrank and declined in number, whereas the diamond-like bases increased. 11e I D /I G ratio of each specimen measured less than 1.8; thus, the crystallized particles were smaller than 2.5 nm, and the DLC film of the specimens belonged to non-crystallized structures.
The I D /I G ratios obtained in the experiment are shown in Table 5.The value for Specimen C1 was 0.484, that for Specimen 1 was 0.585, that for specimen C2 was 0.532, that for Specimen 2 was 0.647, that for Specimen C3 was 1.012, and that for Specimen 3 was 1.483.These results indicated that after cryogenic treatment, the DLC film Raman

Hardness test analysis
In this study, a Vickers hardness tester was used to measure the hardness value of NAK80 mold steel with and without cryogenic treatment; the results are presented in Table 6.A nanoindenter was used to measure the hardness of the DLC film, and the total indentation depth was controlled to less than 100 nm to avoid substrate effect.Five spots were chosen for determining the average values of each specimen; area effect to the samples was avoided.The results obtained matched the Raman intensity ratio, as shown in Table 7.According to the data provided in Table 7, the hardness of NAK80 mold steel without cryogenic treatment was approximately 379.8 HV, and the hardness of that with cryogenic treatment was 381 HV.
The hardness of the specimens after cryogenic treatment was increased by 1-2 HV.The reason for this result is that after cryogenic treatment, the austenite retained in the material can be completely transformed into martensite, and residual stress can be eliminated to enable the texture to achieve relatively greater stability.Therefore, the higher the residual austenite content, the lower the relative hardness.
According to the theory proposed by Stan Veprek, 12 the higher the bond energy among atoms within the layers is, the shorter the bond distance between atoms; comparatively greater hardness can also be achieved.Therefore, in this study, a siliconcarbon bond was smaller in the bond energy and longer among atom lengths compared with a carboncarbon bond.The addition of silicon elements therefore exerts a negative effect on film hardness.
When DLC film is doped in silicon, the sp 3 content of the film's internal structure declines, leading to graphization, which reduces the hardness of the DLC film.
Data in Table 8 show that when the I D /I G value of the DLC film was increased, the sp 2 content increased, the sp 3 content decreased, and the relative hardness was reduced.By contrast, when the I D /I G value of the DLC film was reduced, its sp 2 content declined, the sp 3 content rose, and the relative hardness increased.The hardness of Specimen C1 was 59.34 GPa and the I D /I G value was 0.484.The hardness and strength of the DLC film was most outstanding.
The ranking of the hardness of the coating specimens was C1 > C2> C3> 1 > 2 > 3.This experiment involved using the pin-on-disk wear test, employing a tungsten carbide ball to abrade the specimens.This process was used to calculate the friction coefficient (μ) and wear rate.The wear resistance of each specimen was determined according to the wear rate.

Friction coefficient analysis
The experiment results showed that the friction coefficient of the NAK80 mold steel specimens list the parameters used in this study, and Fig. 1 shows a schematic diagram of the CAPD equipment.In this study, silicon was added to DLC film to enhance the film's wear resistance.

Fig. 1
Fig. 1 Schematic diagram of the CAPD equipment 2.3 Cryogenic treatment During the experiment, cryogenic treatment was performed to stabilize NAK80 mold steel.After quenching and tempering, the specimens were covered with a thin DLC film and slowly cooled to -300 °F at a rate of 0.66 °F per minute.The temperature was maintained for 6 hours and then increased to 300 °F at a rate of 0.33 °F per minute, where it was maintained for 4 hours.The specimens were cooled in the furnace to room temperature to improve the wear resistance of the film.Fig. 2 shows the timetemperature curve for cryogenic treatment of the specimens.

Fig. 2
Fig. 2 Time-temperature curve for cryogenic treatment of the specimens 2.4 Coating structure analysis In this study, the coating structure was analyzed

Preprints
(www.preprints.org)| NOT PEER-REVIEWED | Posted: 7 December 2016 doi:10.20944/preprints201612.0039.v1ranged between 0.85 and 1.2.After DLC coating, the friction coefficient of Specimens 1, 2, and 3 ranged between 0.18 and 0.3 and the friction coefficient of the NAK80 mold steel after cryogenic treatment ranged between 0.94 and 1.15.After cryogenic treatment, the coefficients for Specimens C1, C2, and C3 ranged between 0.05 and 0.24, as shown in Fig. 5.These findings indicate that applying cryogenic treatment to the DLC film can reduce the friction coefficient.The average friction coefficient of the specimens matched the values provided using Raman spectroscopy, as shown in Table 8.Increases in the I D /I G ratio promoted the graphization of the DLC film; thus, a solid lubricating effect was generated to reduce the friction coefficient.

Fig. 5
Fig. 5 Friction coefficient analysis: a specimens without cryogenic treatment; and b specimens with cryogenic treatment 3.4.2Wear rate analysis The experiment results showed that Formulae (1) and (2) can be used to obtain the wear rate of each specimen, as shown in Table 9.The Raman spectroscopy and nanoindentation test results corresponded with the wear rate measurement.The rating of the wear resistance of the specimens was C3> 3> C2> 2> C1> 1> CS> S. A comparison of the wear rate and the Raman intensity ratio (I D /I G ) indicated that when the DLC film I D /I G increased, the wear rate declined, and vice versa.The greater the I D /I G intensity ratio, the higher the sp 2 content, and the higher the degree of graphization, the greater the film graphite content and lubrication.The H/E values obtained through nanoindenter tests and the wear test were used

Fig. 6
Fig. 6 Wear rate of each specimen: a Specimen S; b Specimen CS; c Specimen 1; d Specimen C1; e Specimen 2; f Specimen C2; g Specimen 3; and h Specimen C3 4 Conclusion In this study, cryogenic treatment was applied to NAK80 mold steel with DLC films to investigate whether such treatment increases the wear resistance and surface hardness of DLC films.The results of this study were as follows:

3 .
The wear test revealed that adding silicon to the DLC films increased the graphite content in the film structure.The presence of silicon induced carbon bonding that resembles that of graphite, as confirmed by the increased Raman I D /I G ratio.An increase in the I D /I G and H/E ratios indicates a decrease in the wear rate of the DLC films; conversely, a decrease in the aforementioned ratios indicates an increase in the wear rate of the films.After the films were cryogenically treated, their wear resistance improved.The wear mechanisms of the cryogenically treated and untreated substrates entailed both adhesion and surface fatigue, whereas the DLC films primarily exhibited abrasion.

Table 1 NAK80 compositional analysis Table 2 CAPD process parameters Process 1 Process 2 Process 3
(300 °F) was conducted, and the furnace was cooled to room temperature.The substrate specimens were labeled S, and those subjected to coating were

Table 9 Wear rate and ID/IG parameters
Wear is an abrasive and damaging effect that