فهرست مطالب فرهاد عظیمی فر

The thermal spraying method with the aim of increasing the working life of steel parts is one of the most important solutions in surface engineering to solve the problem of wear.

In this research, NiCrBSi and NiCrBSi-MoS2 coatings were applied on 304 stainless steel by thermal spraying, and then the wear behavior was evaluated at ambient temperature and 500 degrees. Phase investigations were done by X-ray diffraction test. The chemical composition of the coatings was checked with the help of energy dispersive spectrometer analysis. Porosity was investigated with the help of optical microscope and scanning electron microscope images. The hardness of the samples was measured using a microhardness test. In order to check the adhesion and tribological behavior of the coatings, VDI3198 and ASTM-G99 pin-on-disk tests were used, respectively, and finally, the wear mechanism was evaluated using SEM images and EDS analysis of the wear surfaces of the samples.

Phase analysis (XRD) and chemical composition (EDS) showed that the coating has an amorphous and crystalline phase and the most important phases of the coating are nickel-gamma, carbide and boride. Porosity measurement results with the help of image analysis software showed higher porosity of NiCrBSi-MoS2 coating. The results of hardness measurement from the cross-section of the samples indicated an increase in the hardness of the substrate in the presence of coatings, and the addition of MoS2 particles decreased the hardness of the NiCrBSi coating. The coating with MoS2 has better adhesive behavior due to its crystalline structure and better plastic deformation ability. The tribological results indicated the superiority of the NiCrBSi-MoS2 coating due to the appropriate ability of plastic deformation as well as the intrinsic lubrication property (based on the crystal structure) in this test. It was found that at ambient temperature, the samples mainly had lamellar wear mechanism, and at high temperature, oxidation wear as a secondary mechanism helped to destroy the surface.

The addition of MoS2 to NiCrBSi coating caused more porosity, more roughness, less hardness and better wear behavior of the coating. The addition of MoS2 to the coating improved the wear resistance of the coating at high temperature.

One of the most widely used methods to Improving the surface resistance of industrial parts is applying composite coatings such as WC-10Co-4Cr. But this coating, despite its excellent corrosion resistance, does not have a good tribological behavior compared to other tungsten carbide base coatings. For this purpose, in this research, by adding graphite in two amounts of 7% and 14% by weight to WC-10C-4Cr powder and applying coatings by thermal spraying on the steel substrate, their tribological behavior was compared. Investigations were carried out with the help of SEM images, XRD test, roughness measurement and hardness measurement. Pin-on-disk test was also used to evaluate the wear resistance behavior of coatings and substrate. The results showed that despite the relative decrease in hardness and increase in the roughness of the tungsten carbide base coating, graphite leads to a significant improvement in the tribological behavior of the substrate and the tungsten carbide coating. Among the coatings, coating with 7% graphite It had the best wear resistance due to the combination of suitable hardness, low porosity and low friction coefficient. The wear mechanism of the coatings was also investigated with the help of SEM images and EDS analysis. It was found that the dominant mechanism of coatings containing graphite, dellamination and WC-10Co-4Cr coating was adhesive. The most important reason for increasing the tribological properties of coatings in the presence of graphite was the formation of a protective layer of these reinforcing particles in the wear path and around the pin.

Electrolytic plasma oxidation coatings are one of the most important materials used to deal with the wear of aluminum parts due to their compact, sticky and hard nature. In recent years, improving the surface properties of these coatings with the help of adding reinforcement particles has attracted the attention of researchers.

Methods:

 In this study, plasma electrolytic oxidation (PEO) process was applied in two modes, normal and composite, to coat aluminum 7075. To produce the composite coating, the same electrolyte as the normal mode was used with the addition of 12 grams per liter of silicon oxide particles. Characterization of coatings was done with the help of SEM, X-ray diffraction analysis, energy dispersive spectrometer (EDS) analysis, roughness measurement and hardness measurement. The tribological behavior of coatings was investigated with the help of pin-on-disk test and the analysis of the wear mechanism of the samples was done with the help of SEM images and EDS analysis of wear surfaces.

Both coatings had a pancake structure with craters. Also, the drainage channels that lead to the porosity of the coatings were less created on the surface of the composite coating. The normal coating was an oxide coating with an amorphous/crystalline nature, but the composite coating had silicon oxide and mullite crystalline phases in addition to these two phases. The lowest weight loss (highest wear resistance) and friction coefficient were obtained for the composite coating. wear mechanism was also observed delamination wear and oxidation wear for substrate, abrasion, adhesive and oxidation  wear for PEO coating, and fine scratch mechanisms and adhesive mechanism with very little effect for composite coating.

This research investigates the relation between overvaluing of equity and discretionary accruals. The statistical population was taken from listed companies in Tehran Stock Exchange, and sample of 86 listed companies was surveyed. High value of equity provides a strong incentive for managers to report earnings that do not disappoint the market. Jensen (2005) argued the overvalued firms can suffer from adverse consequences because of extreme valuation that encourages managers to act in ways that are detrimental to the long run value of their firms. For example when a firm becomes more overvalued, the pressure to meet increasingly unrealistic earnings targets becomes greater. We use two measures to identify overvalued firms: a) the price-earnings ratio and b) abnormal return. We stratify firms into three classes based on a procedure using both high prior-year P/E ratio and high prior-year abnormal return. Firms in highest class are classified as highly valued and are predicted to have higher future discretionary accruals. In summary, we hypothesize that an unusually high P/E ratio or unusually high prior-year abnormal return provides a manager with strong incentives to meet the implied market expectation for earning. Research hypotheses In this research we posit the following hypotheses, H1: Highly valued firms (firms with both high prior-year P/E ratio and high prior-year abnormal returns) will exhibit higher current year discretionary accruals levels relative to firms with lower valuations. H2: The inverse relationship between operating cash flow and discretionary accruals has higher R square for highly valued firms than firms with lower valuations. In this study we used three procedures for testing of hypothesis: a) Regression models, b) Independent- sample T- Test that compares means for two groups of cases and c) Bivariate Correlation that computes Pearson's correlation coefficient. The results of hypotheses test are represented as follows: First hypothesis has tested in two procedures: 1- Independent sample T-Test: results of comparing average discretionary accruals in highly valued firms and firms with low valuation indicate the average discretionary accrual is higher in highly valued firms than firms with low valuation; therefore, first hypothesis is accepted. 2- Application of regression models: Results of investigating of regression model indicate that the highly valuing variable is confirming. Second hypothesis has tested in two procedures too: 1- Bivariate correlation: in this procedure for testing of second hypothesis the Pearson correlation coefficient has been computed for two groups of firms and results indicate: a) the relation between operating cash flow and discretionary current accruals is inverse and b) quantity of this coefficient is more negative in highly valued firms than firm with low valuation but although the negative coefficients quantity indicates inverse relation in highly valued, it isn’t statistically significant at the 5% level; therefore, the second research hypothesis is not conforming. 2- The regression model: The results of investigating of model coefficients indicate although the negative coefficients quantity of interaction term HV*OCF indicates a strong inverse relation in highly valued, it isn’t statistically significant at the 5% level. Discussion and This paper provides evidence consistent with over valuation hypothesis that predicts that managers of highly valued firms have strong incentives to manage earnings up wards. The evidence suggests that high valuation increases the likelihood of earning management. Our results suggest that prices can drive accruals in contrast to the typical model where accruals drive price. One implication for directors is that they should be particularly conscious of potential earnings manipulation when their firms have extremely high valuation multiples.