Influence of technological parameters on roughness during coating processing

Introduction

Modern processing methods, including electro-pulse treatment and abrasive processing with cooling, significantly enhance the performance of machine parts. These techniques reduce micro-irregularities, decrease friction and wear, and suppress the formation of microcracks. The application of superhard diamond and cubic boron nitride (CBN) grinding further increases hardness and wear resistance, while precision technologies ensure high geometric accuracy. This is particularly critical for parts operating under dynamic loads. Consequently, modern processing methods improve not only part quality but also operational reliability [1 – 3]. At the same time, deviations from prescribed processing modes may cause overheating. Such thermal effects can alter the material structure, promote the formation of quenching cracks, and reduce strength, ultimately leading to warping or even failure of parts during service.

For coating deposition, hardening, and restoration of parts subjected to intensive wear, the use of wear-resistant powder materials such as PG-SR2 and PG-10N-0.2 is considered a promising approach. These materials primarily contain chromium, iron, nickel, and carbide-forming elements (W, Mo, V), which collectively provide high hardness, wear resistance, heat resistance, and corrosion resistance [4 – 6].

Fig. 1 presents the microstructures of coatings based on PG-SR2 + 30 % NPCh1 and PG-SR2 + 75 % NPCh1. Coatings with a lower filler content (30 % NPCh1) exhibit a more homogeneous microstructure. In this case, the microstructure consists of sintered particles of the base component reinforced by filler particles as a result of liquid-phase sintering.

In contrast, coatings with a higher filler content (PG-SR2 + 75 % NPCh1) show the formation of block-like regions composed of relatively large PG-SR2 particles, with finely dispersed NPCh1 filler particles distributed between them.

The experimental results indicate that adding nickel-based fillers (NPCh1, NPCh2, NPCh3) or iron powder PZh6M to PG-SR2 powder in amounts of up to 35 and 25 %, respectively, makes it possible to obtain high-quality coatings with porosity not exceeding 15 %.

Preliminary experiments on induction baking of powder compositions showed that, for all the two-component powder mixtures considered, both their sinterability and their ability to form a firmly bonded coating on the sleeve substrate during induction baking decrease with increasing filler content. When the content of NPCh1, NPCh2, NPCh3, or PG-10N-04 powders exceeds 35 %, or when iron powder PZh6M exceeds 25 % in mixtures with PG-SR2, coating porosity increases markedly. With further increases in filler content, this porosity develops into macroporosity, with pore sizes ranging from 0.5 to 1.5 mm (Fig. 2, a).

Fracture surfaces formed after pin detachment from the baked coating were examined using a scanning microscope. The fractograph of the local fracture zone of the PG-SR2 coating surface is shown in Fig. 2, b. The examined region exhibits a mixed fracture mechanism combining ductile and brittle features. Sharp transition boundaries correspond to brittle fracture, whereas smoother regions indicate ductile deformation. Detachment of the pin along the transition zone (Fig. 2, b) occurs predominantly through ductile fracture, indicating good contact between the coating and the sleeve substrate.

To further improve the properties of baked layers, a surface rolling technology applied after induction centrifugal baking (ICB) was developed. In this approach, restoration is achieved by forming a metallic layer that is then processed by rolling. As a result, surface roughness decreases, the structure becomes more compact, porosity is reduced, and residual compressive stresses are generated. These effects collectively enhance fatigue strength and reduce wear of the working surfaces.

Restoration of the inner surface of a cylinder sleeve by induction baking involves the application of a powder material followed by heating. This technique provides strong adhesion, minimizes thermal impact on the base material, and increases wear resistance (Fig. 3). Nevertheless, subsequent processing of the restored layer remains challenging due to its high hardness, structural heterogeneity, and microstructural changes caused by rapid heating and cooling [5; 6].

A critical stage of final processing that ensures geometric accuracy, minimal surface roughness, and the required mating characteristics with the piston is the grinding of restored sleeves. Rough grinding removes the primary material allowance, whereas finish grinding achieves precise dimensions and refines surface microgeometry. Strict compliance with processing requirements is essential, as this stage directly affects engine service life, wear of the piston assembly, and oil consumption. High-quality grinding is therefore particularly important for engines operating under high load conditions [7 – 9].

In mechanical engineering practice, various approaches are employed to enhance the performance characteristics of abrasive tools. Optimization of tool structure, composition, and operating modes contributes to increased tool durability and improved processing efficiency. Key development directions include the use of advanced abrasives such as diamond and cubic boron nitride, composite, ceramic, and hybrid bonding systems, as well as self-sharpening and porous wheels that promote effective chip removal and heat dissipation. In addition, auxiliary technologies, including electro-pulse and ultrasonic treatments, together with the application of cutting fluid, reduce tool wear and energy consumption [10 – 13].

Internal grinding of engine cylinder sleeves represents a crucial operation for achieving high dimensional accuracy and minimal surface roughness. However, when conventional abrasive wheels (e.g., 25A20PSM18K8B3) are employed, chip adhesion, clogging, and accelerated tool wear often occur, leading to a deterioration in processing accuracy. Previous studies have shown that, during the machining of baked surfaces produced from powder materials such as PG-SR2 containing chromium, iron, and nickel, the issue of wheel loading becomes especially pronounced.

One promising approach to addressing this problem is the use of impregnated abrasive wheels. These wheels are treated with specialized compositions – such as oils, polymers, or metallic compounds – that enhance their functional characteristics. Their main advantages include stable grinding performance, reduced thermal load, diminished clogging, and improved dimensional accuracy, making them particularly suitable for hard and difficult-to-machine materials.

The effectiveness of impregnated wheels is largely attributed to the presence of solid lubricants within their composition. This results in lower contact pressure and cutting forces, reduced processing temperature, and slower wheel wear. Consequently, the use of impregnated abrasive wheels improves both productivity and processing quality long tool life are required.

Materials and methods

Experimental results show that, during internal processing of restored engine sleeves, the use of cutting fluid causes rapid cooling of the chip formation zone. This effect promotes adhesive sticking of nickel and chromium to the abrasive grains, which reduces the cutting ability of the wheel and impairs heat dissipation [14 – 16]. As a consequence, tool wear increases and the temperature in the cutting area rises, potentially leading to the formation of microcracks and defects on the processed surface.

To minimize adhesive sticking, appropriate selection of the abrasive tool, optimization of processing modes, and the use of effective cutting fluids are required. In addition, regular dressing of the grinding wheel is essential [17 – 19].

The use of impregnated abrasive wheels reduces chip adhesion, enhances grain self-cleaning, extends tool life, and lowers the temperature in the contact zone. Impregnators actively participate in the chip formation process by improving friction conditions and reducing cutting forces during the processing of powder materials [20; 21].

The effectiveness of impregnation is determined by the uniformity of impregnator distribution, wheel porosity, and the impregnation technology employed. Impregnated wheels acquire hydrophobic properties that protect them from moisture and extend their storage life [18; 19].

In the present study, three methods were used to process the internal surfaces of restored engine sleeves:

– processing with cutting fluid (3 % emulsion);

– processing using abrasive wheels impregnated by the standard method [18; 19];

– processing using abrasive wheels impregnated by the proposed method.

In the proposed impregnation method, the abrasive wheel is mounted on spindle 7 of the impregnation device (Fig. 4) and immersed in bath 6 containing the impregnation solution, allowing free filling of the wheel pores. After removal, the wheel is rotated at 1000 rpm to eliminate excess solution. This procedure ensures uniform distribution of the impregnator and reduces the likelihood of wheel loading by chips [18; 22].

Results and discussion

Surface roughness of the processed surfaces was evaluated as a function of the processing modes. The results show that the dominant factor influencing surface roughness is the rotation speed of the part. Curve 3 in Fig. 5 corresponds to processing with a conventional abrasive wheel using cutting fluid, whereas curve 2 represents processing with an abrasive wheel impregnated by the standard method. Curve 1 demonstrates a pronounced reduction in surface roughness when the proposed impregnation method is applied, yielding the best overall performance.

Abrasive wheels impregnated by the proposed method reduce cutting forces and plastic deformation, which contributes to lower surface roughness. In addition, these wheels are less susceptible to wheel loading, and grinding chips are efficiently removed from the contact zone.

Experiments investigating the influence of processing modes – namely, the part rotation speed V_p, the grinding wheel speed V_s, and the longitudinal feed S_f – showed that increasing V_p and S_f leads to an increase in surface roughness for all processing methods. As V_p increases, the R_a value rises for all investigated modes (Fig. 5). The minimum roughness values are characteristic of mode 1, corresponding to the abrasive wheel impregnated by the proposed method, whereas mode 3 (a conventional abrasive wheel used with cutting fluid) produces the highest R_a values over the entire range of part rotation speeds.

An increase in the grinding wheel rotation speed V_s results in an overall decrease in R_a for all investigated modes (Fig. 6). The minimum roughness values are again achieved under mode 1, while mode 3 is characterized by the highest R_a values throughout the studied range of wheel rotation speeds.

As shown in Fig. 7, increasing the longitudinal feed S_f leads to a rise in R_a for all processing modes. The lowest surface roughness is observed for mode 1, whereas mode 3 yields the highest R_a values over the entire range of longitudinal feed rates.

The differences between the curves presented in Figs. 5 – 7 clearly indicate the significant influence of processing conditions on the formation of surface microrelief.

The experimental results further demonstrate that, during internal processing of engine sleeves restored using powder materials, the application of impregnated abrasive wheels reduces surface roughness by a factor of 1.5 – 1.8. Thus, the proposed impregnation method provides a substantial improvement in the processing quality of restored engine sleeves.

Fig. 8 presents micrographs of polished sections obtained from worn cylinder sleeves of the 6Ch17/14 marine diesel engine that had reached their inter-repair service life. In terms of graphite distribution within the metallic matrix, the microstructure of the worn sleeve corresponds to grade Gd-6. The graphite morphology in the cylinder sleeves is predominantly dendritic–rosette, with the presence of rounded graphite inclusions. The metallic matrix of the cylinder sleeves (Fig. 8, a) corresponds to pearlite of grade P–P95, with the ferrite content not exceeding 1.0 – 1.5 % of the polished section area.

Figs. 8, e and f show the microstructure of the cylinder sleeves after induction baking of the powder coating. In the coating zone, graphite inclusions in the cast iron correspond to grade Gd-5. The graphite morphology remains dendritic–rosette, with a slight increase in the fraction of rounded graphite.

After subsequent mechanical processing, the microstructure of the coated sleeves closely resembles that of the worn sleeve. Analysis confirmed that the structure of the metallic matrix of the sleeves remains within the requirements specified by GOST.

Conclusions

Increasing the part rotation speed during grinding extends the contact length between the abrasive grains and the processed surface. This results in higher residual roughness heights, enhanced thermal effects, and modifications of the mechanical properties of the surface layer.

By contrast, increasing the grinding wheel rotation speed raises the frequency of contacts between the wheel and the part, thereby involving a larger number of abrasive grains in the material removal process. This leads to a more uniform material removal and, consequently, to a reduction in surface roughness.

Kinematic analysis of the grinding process confirms that higher workpiece rotation speeds increase the contact length of abrasive grains, which in turn causes an increase in residual roughness heights. At the same time, higher grinding wheel rotation speeds are associated with an overall decrease in the roughness of the inner surface of the sleeve.

Experimental investigations further demonstrate that, during internal processing of engine sleeves restored using powder materials, the use of impregnated abrasive wheels reduces surface roughness by a factor of 1.5 – 1.8.