The aim of this study is to exhibit the mutual connection between surface and in depth hardness values in the case of surface-treated metal samples with inhomogeneous hardness distribution in the surface layer. The reason for surface treatments of metal alloys is most commonly to increase the hardness and wear resistance at the surface. Case depth, as a result of surface treatment and the in-depth hardness distribution, can be determined by measuring the hardness of a section perpendicular to the treated surface and by metallographic examination. The result of heat treatment can also be checked rapidly by surface hardness testing. Surface hardness carries only indirect information regarding case depth and hardness distribution. Surface and cross-sectional hardness can be related to the mathematical modeling of the plastic zone developing in the indentation process. The mathematical model applied in this study allows the conversion of the surface hardness function into the in-depth hardness function and vice-versa. The calculation method presented was validated by analyzing the hardness data of nitrocarburized samples of various case depths. The validation result proves that cross-sectional hardness distribution can be adequately estimated by surface hardness data in the case of a surface layer with monotonically decreasing hardness distribution.

The porosity of die-cast aluminum alloys is a determining factor for the quality of the product. In this paper, we studied the porosity of a selected part of a die-cast AlSi9Cu3(Fe) compressor part by computer tomography and metallography. In the case of this part, the achievable resolution by CT, a non-destructive testing method, was 30 μm—this method could not detect smaller cavities. Based on metallographic analysis, the percentage of defects larger than 30 μm ranges from 10 to 30% of the total number of defects, which represents 75–95% of the defective area (area ratio). Impregnation with methacrylate resin (used to seal cavities to prevent leakage) can be detected with UV-illuminated optical microscopic examination on metallographically prepared specimens. As confirmed by scanning electron microscopy, partial filling and partial impregnation can occur in a system of shrinkage cavities.

In continuously cast semi-finished products, fluid flow results in macrosegregation patterns. There are two reasons for fluid flow during solidification: density differences in the liquid phase and outer forces or constraints, which mix, suck or squeeze the liquid inside. The calculation method described in this paper can be used to analyze the effects of fluid flow caused by outer constraints and to estimate the expected macrosegregation level.

This study examines dry sliding friction between a hydrogenated amorphous carbon (a-C:H) diamond-like carbon (DLC) coating and a polypropylene homopolymer (PPH). The coating was deposited on a quenched and tempered 42CrMo4 steel specimen (39 ± 1 HRC). Friction tests were performed at three sliding speeds (v = 5, 50, and 500 mm/min) and three surface pressures (p = 2, 4, and 6 MPa). From the recorded friction-force curves, the static friction coefficient (μ0) was taken as the peak value at the onset of motion, while the dynamic friction coefficient (μdyn) was evaluated in the near steady-state (gross sliding) region. Both μ0 and μdyn increased with sliding speed: μ0 rose from about 0.24 to 0.33 and μdyn from about 0.22 to 0.32 in the investigated range. Surface pressure had a smaller effect: μ0 showed no clear pressure trend, whereas μdyn increased with pressure at a given speed. A two-factor analysis confirms that sliding speed is the dominant factor for both μ0 and μdyn, and it also indicates a weak non-linear speed dependence in this parameter range. Overall, the friction coefficient of the PPH–DLC pair remained approximately within 0.20–0.35 for v = 5–500 mm/min and p = 2–6 MPa.

The tribological behavior of metal–polymer friction pairs depends to a large extent on their surface roughness. In this study, we analyze the friction of C45 (1.0503) steel and polypropylene homopolymer (PPH) in terms of the surface roughness characteristics of the material pair subjected to friction. The results of the friction experiments described in the article showed that during the friction process, the investigated roughness parameters of the steel specimen (Ra – average surface roughness, Rz – roughness height or ten-point height, Rp – maximum peak height, Rv – maximum valley depth) remained essentially unchanged, while the same roughness parameters of the polymer surface significantly changed and adapted to the roughness characteristics of the steel. Ellipsometric measurements also confirmed the presence of a 20–30 nm thick polymer transition film on the steel surface, and there were parts of steel on the polymer surface as well. The obtained results indicate that in the case of metal–polymer friction pairs, the change in surface roughness is asymmetric (the surface roughness of the harder material shapes the softer surface). In contrast, material transfer is bidirectional. These must be taken into account in friction modeling and wear prediction.