Influence of Chromium Concentration on the Abrasive Wear of Ni-Cr-B-Si

metals

Article Inﬂuence of Chromium Concentration on the Abrasive Wear of Ni-Cr-B-Si Coatings Applied by Supersonic Flame Jet (HVOF)

Mara Kandeva 1,2, Zhetcho Kalitchin 3,* and Yana Stoyanova 1

1 Tribology Center, Faculty of Industrial Engineering, Technical University–Sofia, 8 Kl. Ohridski Blvd, 1757 Sofia, Bulgaria; kandevam@gmail.com (M.K.); yast@tu-sofia.bg (Y.S.) 2 South Ural State University, 76 Prospekt Lenina, 454080 Chelyabinsk, Russia 3 SciBuCom 2 Ltd., P.O. Box 249, 1113 Sofia, Bulgaria * Correspondence: [email protected]

Abstract: This research work studies the characteristics of wear and wear resistance of composite powder coatings, deposited by supersonic ﬂame jet (HVOF), which contain composite mixtures Ni-Cr-B-Si having different chromium concentrations—9.9%, 13.2%, 14%, 16%, and 20%, at one and the same size of the particles and the same content of the remaining elements. The coating of 20% Cr does not contain B and Si. Out of each powder composite, coatings have been prepared without any preliminary thermal treatment of the substrate and with preliminary thermal treatment of the substrate up to 650 ◦C. The coatings have been tested under identical conditions of dry friction over a surface of solid, ﬁrmly-attached, abrasive particles using tribological testing device “Pin-on-disk”. Results have been obtained and the dependences of the hardness, mass wear, intensity of the wearing process, and absolute and relative wear resistance on the Cr concentration under identical conditions

of friction. It has been found out that for all the coatings the preliminary thermal treatment of the substrate leads to a decrease in the wear intensity. Upon increasing Cr concentration the wear

Citation: Kandeva, M.; Kalitchin, Z.; intensity diminishes and it reaches minimal values at 16% Cr. In the case of coatings having 20% Stoyanova, Y. Inﬂuence of Chromium Cr concentration, the wear intensity is increased, which is due to the absence of the components B Concentration on the Abrasive Wear and Si in the composite mixture, whereupon no inter-metallic structures are formed having high of Ni-Cr-B-Si Coatings Applied by hardness and wear resistance. The obtained results have no analogues in the current literature and Supersonic Flame Jet (HVOF). Metals they have not been published by the authors. 2021, 11, 915. https://doi.org/ 10.3390/met11060915 Keywords: HVOF coatings; chromium; tribology; abrasion wear

Academic Editor: Jianqiang Wang

Received: 20 April 2021 1. Introduction Accepted: 25 May 2021 Published: 4 June 2021 The basic priorities of contemporary engineering science and practice refer to en- hancement of energy effectiveness and functionality of the industrial systems in harmonic

Publisher’s Note: MDPI stays neutral coexistence with clean environment, preservation of natural resources, and improving of with regard to jurisdictional claims in the quality of life. These priorities are connected with the genesis of tribology as contem- published maps and institutional afﬁl- porary science and technology for contact processes of friction, wear, and greasing in the iations. technical systems [1–3]. The lowering of the wear intensity in the machines appears to be the central task of tribology tribological technologies. It is the reason for more than 85% of the machine failures, huge expenses of materials, and human resources for spare parts, consumables, and expenses for maintenance in the process of operation. The decrease in the wear intensity results not only in lower ﬁnancial expenses, but also decrease in Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. extraction of raw materials from the natural environment, which is directly connected with This article is an open access article the equilibrium in eco-systems [4–10]. distributed under the terms and The abrasive and erosive wear are the most dangerous types of wear in the machines conditions of the Creative Commons for ore mining, road construction, agricultural technique, energy production, transportation, Attribution (CC BY) license (https:// aircraft building, and space technology. They are the reason for more than 50% of all the creativecommons.org/licenses/by/ technical failures due to wear of machine components and the expenses due to it in the 4.0/). world reach 1–4% of the gross domestic product GDP of the developed industrial countries

Metals 2021, 11, 915. https://doi.org/10.3390/met11060915 https://www.mdpi.com/journal/metals Metals 2021, 11, 915 2 of 17

in the world. The problem of abrasive and erosive wear resistance of tribological materials is of exceptional importance and actuality, which is evidenced by the fact that there exist 17 active standards ASTM, concerning the abrasive wear and at least 4 standards connected with the erosion wear. It is of special importance for the high wear resistance of tribology materials to combine high hardness and plasticity, which are mutually self-excluding aspects and they cannot be achieved by most of the conventional tribology technologies [11–24]. The composite powder coatings, deposited by means of supersonic flame jet-HVOF (High Velocity Oxy-Fuel) are a new generation of tribo-materials, allowing to achieve a wide range of combining mechanical and tribological characteristics—high adhesion strength, density, hardness, wear resistance, and corrosion stability. The powder particles obtain high kinetic and thermal energy in the flame jet, passing over into semi-plastic and/or plastic state in the form of droplets or particles, and they interact with the substrate being deformed, whereupon they form thin lamellas. Upon their collision with roughness of the substrate the particles’ droplets are being cooled down, forming adhesion bonds with the surface, and cohesion bonds between each other leading to generation of the laminar structure of the composite coating. Due to the high velocity and low contact time of the particles with oxygen under definite conditions they do not form oxides, which, in fact, is a prerequisite for good tribological properties of the coatings [25,26]. HVOF-coatings are the object of intensive investigations and they are continuously being improved [27–41]. The results of studies of the authors and those of other researchers show that the mechanical and tribological characteristics of the HVOF-coatings, depend not only on the parameters of the technological regime of deposition, but also to a great extent they depend on the chemical composition and on the concentrations of the chemical elements in the powder composite material, the size of the particles, and on the temperature of the substrate. The results also depend on the type and the dimensions of the contact (point type, linear type, planar type) [29,31,35]. The papers [35,36] present interesting results concerning the ceramic coatings with aluminum oxide matrix (Al2O3) with matrix of 3% titanium oxide (TiO2) and zirconium oxide (ZrO2) matrix with 30% calcium oxide (CaO) on the basis of non-alloy structural steel of the S235JR class. The buffer layer is obtained on the basis of Ni-Al-Mo powder. The complex investigations including metallographic tests, phase composition, microhardness, adhesion of coating to the base, resistance to abrasion, and heat shock show that these coatings have high abrasive and erosion wear resistance and resistance towards thermal impact load. In the paper [37] the results of the investigation of the mechanical properties of aluminum coatings are presented by the same Authors. The coatings are strengthened with carbon materials with nano-dimensions: carbon nanotubes Nanocyl NC 7000 (0.5 w.% and 1 w.%) and (carburate 0.5 w.%). It has been proved that the applied coatings with nano-carbon particles, will be an effective alternative of coatings applied with laser technology and might be additionally improved and successfully applied in the automotive, aviation, and space technology. According to the specialized literature the most widely used in the practice high quality powder metal composites, known as “super alloys”, can be divided into two large groups: powder composites based on nickel (nickel-chromium-boron-silicon (Ni-Cr- B-Si)) and powder composites based on cobalt (tungsten-cobalt-chromium alloys). The chromium at high temperatures is involved into contact interactions with the carbon, silicon, and boron, whereupon it forms new inter-metallic structures. It forms with the carbon solid high-melting structure (Cr3C2) having a green color, which is not dissolved in acids and which attributes to the coating high corrosion stability. Chromium forms with the oxygen some oxides (CrO3, Cr2O3), which are characterized by high level of hardness and fragility [33,34]. In the paper [40], the influence of the dimensions of the abrasive particles has been presented on the abrasive stability on the coatings of 40WC/Co, applied by three technological regimes: high velocity oxygen fuel (HVOF), high energy plasma spray (HEPS), and high velocity plasma spray (HVPS) with the use of nanostructured experimental powders. Metals 2021, 11, x FOR PEER REVIEW 3 of 19

high velocity plasma spray (HVPS) with the use of nanostructured experimental powders. The most important results are: first—the coatings applied by HVOF have the best wear Metals 2021, 11, 915 3 of 17 resistance, while the HVPS coatings have the worst wear resistance; and second—the greater the size of the abrasive particles, the lower the abrasion resistance of the coatings. Thermospray coatings based on tungsten, nickel, and their compounds are widely The most important results are: first—the coatings applied by HVOF have the best wear usedresistance, in extreme while operating the HVPS conditions coatings have due the to worsttheir wearhigh resistance;resistance and to second—thewear and oxidation at highgreater temperatures the size of the [39–41]. abrasive Work particles, [39] the is lower an in-depth the abrasion study resistance of the of influence the coatings. of process primers onThermospray friction, abrasion coatings basedresistance, on tungsten, and corrosion nickel, and of their U.Co compounds coatings are deposited widely on alu- minumused alloy in extreme AA 7050 operating by HVOF conditions process. due to their high resistance to wear and oxidation atThe high present temperatures publication [39–41]. represents Work [39] is the an in-depthresults from study an of theinvestigation influence of processabout the influ- primers on friction, abrasion resistance, and corrosion of U.Co coatings deposited on ence aluminumof chromium alloy AAconcentration 7050 by HVOF on process. the abrasion wear of Ni-Cr-B-Si coatings, deposited by meansThe of presentsupersonic publication flame represents jet on a thesubstr resultsate from with an and investigation without about thermal the influence treatment of the substrate.of chromium concentration on the abrasion wear of Ni-Cr-B-Si coatings, deposited by means of supersonic flame jet on a substrate with and without thermal treatment of 2. Materialsthe substrate. and Technology 2.The Materials investigated and Technology coatings are obtained in the following consecutive steps: preparation of theThe powder investigated compositions, coatings are obtainedpreparation in the followingof the substrate, consecutive application steps: preparation of the coatings withof HVOF the powder technology, compositions, measurement preparation of ofthe the thickness substrate, and application the hardness of the coatings of the obtained with HVOF technology, measurement of the thickness and the hardness of the obtained coatings, preparation of the samples for testing of the abrasive wear, and calculation of coatings, preparation of the samples for testing of the abrasive wear, and calculation of the the characteristiccharacteristic of of the the wear wear (Figure (Figure1). 1).

Figure 1. Scheme of preparation and research of HVOF-coatings. Figure 1. Scheme of preparation and research of HVOF-coatings. Ten types of coatings were prepared on the basis of ﬁve composite powder mixtures withTen nickeltypes matrix,of coatings united were in ﬁve prepared series A, on B, C,the D, basis E (Table of 1five). The composite serial composition powder mixtures withA, nickel B, C, Dmatrix, containing united different in five concentration series А, ofB, Chromium—9.9%,C, D, E (Table 1). 13.2%, The 14%,serial 16%, composition and А, approximately the same composition of the other chemical elements—Si, B, Fe, Cu, Mo, etc. B, C,The D containing composite mixture different of series concentration E (sample No of 9 andChromium—9.9%, No 10) contain 20% 13.2%, Cr and 80%14%, Ni, 16%, i.e., and ap- proximatelyin it are not the included same composition other elements, of present the other in the chemical compositions elements—Si, A, B, C, D, respectively. B, Fe, Cu, Mo, etc. The compositeFrom each mixture of the series, of series we applied Е (sample two types Noof 9 coatings;and No without 10) contain preliminary 20% thermicCr and 80% Ni, processing of the substrate (cold HVOF process); and with preliminary heating of the i.e., in it are not included other elements, present in◦ the compositions А, B, C, D, respectively.substrate in thermic chamber at a temperature of 650 C for 60 min. The coatings with thermic processing of the substrate are denoted with PHS. FromAll each the coatings of the series, have been we depositedapplied two on a ty substratepes of coatings; of one and without the same preliminary material- ther- mic processingsteel of chemical of the composition: substrate C—0.15%;(cold HVOF S—0.025%; process); Mn—0.8%; and with P—0.011%; preliminary Si—0.21%; heating of the substrateCr—0.3%; in thermic Ni—0.3% chamber and hardness at a 193.6–219.5 temperature HV. of 650 °С for 60 min. The coatings with thermic processingThe particles of in allthe the subs powdertrate composites are denoted have with one andPHS. the same size—45 ± 2.5 µm. Before placing of the powder composite inside the system for thermal spraying, it is heated for 30 min at temperature 150 ◦C in thermal chamber for removing the moisture and the other adsorbed organic molecules.

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Table 1. Description, chemical composition, hardness, and thickness of the tested coatings.

Coating Chemical Hardness Thickness Series Sample Description Designation Composition, wt. % HRC µm Coatings without heat 1 NiBSi-9.9Cr Cr: 9.9; Si:3.1; B:1.7; 55–56 410–415 treatment of the substrate A Fe:3.2; C: 0.35; Mo: 3; NiBSi- Coatings with heat Cu:3; 2 58–59 400–412 9.9Cr:PHS treatment of the substrate Ni Balance Coatings without heat 3 NiBSi-13.2Cr Cr: 13.2; Si: 3.98; B: 56–57 395–400 treatment of the substrate B 2.79; Fe: 4.6; Co: 0.03; NiBSi- Coatings with heat C: 0.63; 4 59–60 408–414 13.2Cr:PHS treatment of the substrate Ni: Balance Coatings without heat 5 NiBSi-14Cr Cr: 14; Si:4.2; B:2,9; 57–58 395–406 treatment of the substrate C Fe:4,6; C:0.6; Mo:2,5; NiBSi- Coatings with heat Cu:2,4; 6 59–60 400–405 14Cr:PHS treatment of the substrate Ni: Balance Coatings without heat 7 NiBSi-16Cr Cr:16; Si:4; B:3.4; 58–59 394–405 treatment of the substrate D Fe:2.7; C:0.6; Mo:3; NiBSi- Coatings with heat Cu:3; 8 61–62 404–410 16Cr:PHS treatment of the substrate Ni: Balance Coatings without heat 9 Ni80-20Cr 54–55 400–408 treatment of the substrate E Cr: 20; Ni: 80 Coatings with heat 10 Ni80-20Cr:PHS 57–60 393–403 treatment of the substrate

In order to increase the adhesion strength of the coatings, the substrate is heated preliminarily in three stages: cleaning, erosion with abrasive particles (blasting), and mechanical treatment. The cleaning is aimed at the removal of mechanical contaminants, adsorbed organic molecules, moisture, and other components, and it is carried out using a solvent. The extraction of the adsorbed gas molecules and elements in the depth of the surface layer is achieved by burning of the surface of the substrate with a ﬂame to reach 100 ◦C at a distance of the nozzle 40 mm and at an angle of 45◦ or with vapor spraying device. After this operation, again the surface is cleaned with a solvent. Upon erosion of the surface of the substrate (blasting) one can achieve a deﬁnite level of roughness of the substrate, which is of essential importance for the level of the adhesion strength of the coating. We used abrasive material “Grit”, in accordance with the requirements of the standard ISO 11126, having granular composition of the abrasive material in the following percentage ratio: 3.15 ÷ 1.4 mm—9.32%; 1.63 ÷ 0.5 mm—16.4%; 1.4 ÷ 1.0 mm—15.8%; 1.0 ÷ 0.63 mm—39.6%; 0.5 ÷ 0.315 mm—9.32%; 0.315 ÷ 0.16 mm— 9.32%; particles having sizes below 0.15 mm of the various fractions—up to 100% of the following chemical compounds: SiO2—41%, combined in the form of silicates; AlO—8.3%, MgO—6.6%, CaO—5.5%, and MnO—0.4%. The system for blasting has the following technical parameters: input pressure 8 atm; operating pressure in the nozzle—4 atm; diameter of the nozzle 7 mm; distance between the nozzle and the surface—30 mm; angle of interaction of the jet with the surface—90◦. The coatings have been deposited using the device MICROJET + Hybrid, which makes use of fuel mixture of acetylene and oxygen. The parameters of the technological regime of deposition of the coatings are listed in Table2. Metals 2021, 11, 915 5 of 17

Table 2. Technological regime parameters for HVOF coating deposition.

Parameter Technological Regime Propylene/oxygen ratio 55/100% Particle velocity 1000 m/s Spraying distance 100 mm Impact angle 90◦ Air pressure from compressor 5 bar

N2 pressure in the proportioning device 4 bar Powder feed rate 22 g/min

In case of deposition of coatings without thermal treatment (cold HVOF process), the surface of the substrate is heated using flame having temperature up to 200 ◦C, which is measured by Laser infrared thermometer INFRARED with an error of 0.5 ◦C. The coating is deposited in several layers. In the case of the first layer the nozzle is situated at an angle of 45◦ and at a distance from the substrate 10 mm, while in the consecutive layers—at a distance of 25 mm. Coatings have been prepared having thickness within the range from 393 µm up to 415 µm. The thickness of the coatings is measured by a portable device Pocket Leptoskop 2021 Fe in 10 points on the surface and the mean arithmetic value is taken (Table1). The accuracy of the apparatus is 1 µm. After polishing all surfaces, the coatings have the same roughness Ra = 0.450–0.455 µm, which is measured by recording the profile diagram using profile metering device “TESA Rugosurf 10-10G” with an error of 0.005 µm. The hardness of the coatings is measured by hardness-metering device “Bambino” based on the scale of Rockwell (HRC) taking the mean arithmetic value out of three measurements for each sample in order to eliminate some possible effects of segregation.

3. Experimental Procedures The abrasive wear of the coatings is studied under conditions of dry friction during sliding along the surface with firmly attached abrasive particles. The methodology consists in measurement of the mass wear of the coatings after a definite pathway of friction (number of cycles) under set permanent conditions-loading, sliding velocity, kinds of the abrasive, temperature of the environment. The mass of the samples before and after a definite pathway of friction is measured by electric balance WPS 180/C/2 with an accuracy of 0.1 mg. In each experiment, with each sample, the abrasive surface is replaced and prior to each measurement the sample is cleaned, removing mechanical and organic particles, thereafter it is dried up using ethyl alcohol in order to prevent the electrostatic effect. After measuring of the mass wear the wear process characteristics are calculated- reduced intensity of the wear process, the absolute or the relative wear resistance. The mass wear in mg is obtained as the difference between the initial mass of the sample mo and its mass mi after a definite number of cycles of friction:

m = mo − mi (1)

The wear rate of the wear process ir represents the mass wear m of the coating per unit of loading P and per unit of path length L of friction. It is measured in mg/Nm and it is estimated by the formula: m i = (2) r PL Metals 2021, 11, x FOR PEER REVIEW 6 of 19

The mass wear in mg is obtained as the difference between the initial mass of the sample 𝑚 and its mass 𝑚 after a definite number of cycles of friction:

𝑚=𝑚 −𝑚 (1)

The wear rate of the wear process 𝑖 represents the mass wear m of the coating per unit of loading Р and per unit of path length L of friction. It is measured in mg/Nm and it is estimated by the formula: 𝑚 𝑖 = (2) Metals 2021, 11, 915 𝑃𝐿 6 of 17

The absolute abrasive wear resistance 𝐼 is represented as the reciprocal value of the reduced wear rate and it has dimension Nm/mg, i.e., The absolute abrasive wear resistance Ir is represented1 𝑃𝐿 as the reciprocal value of the reduced wear rate and it has dimension Nm/mg,𝐼 = i.e., = (3) 𝑖 𝑚 1 PL The relative wear resistanceI r𝑅= is= dimensionless quantity and it represents(3) the ratio i m r between the wear resistance of the tested sample 𝐼 and the wear resistance of a sample, takenThe as relative a standard wear resistance𝐼, determinedRij is dimensionless during identical quantity regimes and it represents of friction, the i.e., ratio i between the wear resistance of the tested sample Ir and the wear resistance of a sample, j taken as a standard Ir, determined during identical regimes𝐼 of friction, i.e., 𝑅 = (4) 𝐼 Ii = r Rij j (4) The abrasive wear is studied using Itribologicalr tester “Pin-on-disc” in case of plane- like contact using the functional scheme, shown in Figure 2. The studied sample with The abrasive wear is studied using tribological tester “Pin-on-disc” in case of plane- coating 1 (pin) is firmly attached in the holder 2 of the loading head 8, in such a way that like contact using the functional scheme, shown in Figure2. The studied sample with coatingthe frontal 1 (pin) surface is ﬁrmly of attached the sample in the holder is in contact 2 of the loading with the head abrasive 8, in such surface a way that 3, thefixed to a hori- frontalzontal surface disc 4. of The the disc sample 4 is is driven in contact by withthe electric the abrasive motor surface 6 and 3, it ﬁxed is rotating to a horizontal around its central discvertical 4. The axis disc at 4 isconstant driven byangle the electricvelocity. motor The 6 andnormal it is rotatingloading around pressure its central Р is adjusted by verticalmeans axis of the at constant lever system angle velocity. in the The center normal of loadingthe contact pressure plateP is between adjusted by the means sample and the ofabrasive the lever surface. system in The the pathway center of the length contact of plate friction between as a thenumber sample of and cycles the abrasive (N) is selected and surface. The pathway length of friction as a number of cycles (N) is selected and then measuredthen measured by the turnover by the turnover number metering number device meteri 7.ng The device abrasive 7. surfaceThe abrasive 3 is modeled surface 3 is mod- byeled impregnated by impregnated corundum corundum P 320 of hardness P 320 of 9.0 ha onrdness the scale 9.0 ofon Moos, the scale whereupon of Moos, the whereupon requirementthe requirement of the standard of the standard for minimum for 60%minimum higher hardness 60% higher of the hardness abrasive is of observed the abrasive is ob- withserved respect with to respect that of the to surfacethat of layer the surface of the tested layer materials. of the tested materials.

FigureFigure 2. 2.Schematic Schematic diagram diagram of abrasive of abrasive wear testing wear ontesting pin-disc on tribometer.pin-disc tribometer.

TheThe investigation investigation of allof theall coatingsthe coatings has been has carriedbeen carried out using out a using set of thea set follow- of the following ing parameters of the regime of friction: loading 4.5 N, nominal contact surface area parameters of the regime of friction: loading 4.5 N, nominal contact surface area 2.25 × 10−6 2.25 × 10−6 m2; nominal contact pressure 2.0 N/cm2, sliding velocity of 0.155 m/s; type of 2 2 them abrasive; nominal surface contact Corundum pressure P 320,2.0 temperatureN/cm , sliding of the velocity environment of 0.155 21 ◦ C.m/s; type of the abrasive surface Corundum Р 320, temperature of the environment 21 °С. 4. Experimental Results and Discussion Applying the above-described methodology and the device experimental results have been obtained for the mass wear process, the wear rate, and the absolute and the relative wear resistance for all the studied coatings, listed in Table1. In Table3, we give the results of the mass wear rate at a friction path 20, 40, 60 and 80 m . Metals 2021, 11, 915 7 of 17

Table 3. Abrasive wear and wear rate of tested coatings.

Number of Cycles 100 200 300 400 Coating Sample Sliding Distance, m Designation 20 40 60 80 Mass Wear, [mg]//Wear Rate, [mg/Nm] 4.7 8.5 12.7 15.3 1 NiBSi-9.9Cr 5.22 × 10−2 4.71 × 10−2 4.71 × 10−2 4.24 × 10−2 3.2 5.8 11.2 12.6 2 NiBSi-9.9Cr:PHS 3.56 × 10−2 3.22 × 10−2 4.15 × 10−2 3.51 × 10−2 4.4 7.9 10.6 12.3 3 NiBSi-13.2Cr 4.89 × 10−2 4.4 × 10−2 3.93 × 10−2 3.42 × 10−2 3.5 7.1 9.8 10.4 4 NiBSi-13.2Cr:PHS 3.89 × 10−2 3.96 × 10−2 3.62 × 10−2 2.89 × 10−2 4.0 6.0 7.6 10.5 5 NiBSi-14Cr 4.44 × 10−2 3.33 × 10−2 2.82 × 10−2 2.91 × 10−2 3.1 4.2 5.8 7.6 6 NiBSi-14Cr:PHS 3.44 × 10−2 2.33 × 10−2 2.16 × 10−2 2.11 × 10−2 1.5 2.2 2.8 3.3 7 NiBSi-16Cr 1.67 × 10−2 1.22 × 10−2 1.04 × 10−2 0.91 × 10−2 0.9 1.2 1.6 1.8 8 NiBSi-16Cr:PHS 1.0 × 10−2 0.67 × 10−2 0.58 × 10−2 0.49 × 10−2 2.9 5.6 7.6 10.4 9 Ni80-20Cr 3.22 × 10−2 3.11 × 10−2 2.82 × 10−2 2.89 × 10−2 1.8 4.1 6.3 9.1 10 Ni80-20Cr:PHS 2.0 × 10−2 2.22 × 10−2 2.33 × 10−2 2.51 × 10−2

In accordance with the data in Table3 the kinetic curves have been plotted in regard to the mass wear for all coatings with and without thermal treatment of the substrate, represented in the Figures3–7. Each plotted graph represents regression equations of the wearing process as a function of the length of the friction pathway m = m(L) and the value of the wear rate ir at friction pathway length L = 80 m. It is seen, that in the case of dry abrasive friction, the dependence of the mass wear as a function of the sliding pathway has linear character for coatings with and without thermal treatment of the substrate. The second observation in the analysis of these curves refers to the fact that the wear of all coatings having thermal treatment of the substrate is less than the wear of the coatings without any thermal treatment of the substrate. Figure8 represents graphically the dependence of the wear rate on the concentration of chromium for coatings without thermal treatment of the substrate and for coatings with thermal treatment of the substrate for one and the same friction pathway length L = 80 m. The curves have non-linear character with a clearly expressed minimum of the wear rate at 16% concentration of Cr for coatings with and without thermal treatment. In the ﬁrst section, in which the chromium concentration is changing within the range from 9.9% to 16% upon increasing of the chromium concentration, the wear rate decreases down to reaching a minimal value at chromium concentration 16%, respectively, for coatings −2 without thermal treatment of the substrate ir = 0.91 × 10 mg/Nm and for coatings with −2 thermal treatment of the substrate ir = 0.49 × 10 mg/Nm. In the second section, at higher chromium concentration (20%) (coatings Ni80-20Cr and Ni80-20Cr:PHS), the wear rate is increasing sharply. In spite of the higher chromium concentration the increase in the wear is due to the absence of the elements B, Si, Cu, and the others, which are contained in the other coatings. These elements in the process of contact interaction of the ﬂame jet with the substrate at the high temperature are forming inter-metallic compounds with the chromium, which lead to decrease in the wear rate. Metals 2021, 11, x FOR PEER REVIEW 8 of 19

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20 Coating NiBSi-9.9Cr i = 4.24 x 10-2mg/Nm 20 r Coating NiBSi-9.9Cr:PHS (R² = 0.9918)-2 16 Coating NiBSi-9.9Cr ir = 4.24 x 10 mg/Nm Coating NiBSi-9.9Cr:PHS 16 (R² = 0.9918) 12 m = 0.193.L + 0.52 12 m = 0.193.L + 0.52 8 -2 ir = 3.51 x 10 mg/Nm

Mass wear, mg wear, Mass 8 i = 3.51(R² =x 100.9762)-2 mg/Nm 4 r Mass wear, mg wear, Mass m = 0.166xL(R² - 0.08 = 0.9762) 4 m = 0.166xL - 0.08 0 0 0 20406080 0 20406080Sliding distance, m Sliding distance, m Figure 3. Mass wear vs. sliding distance for NiBSi-9.9Cr and NiBSi-9.9Cr: PHS coatings. Figure 3. Mass wear vs. sliding distance for NiBSi-9.9Cr and NiBSi-9.9Cr: PHS coatings. Figure 3. Mass wear vs. sliding distance for NiBSi-9.9Cr and NiBSi-9.9Cr: PHS coatings. 15 i = 3.42 x 10-2mg/Nm 15 Coating NiBSi-13.2Cr r i (R² = 0.9718)-2 12 Coating NiBSi-13.2CrNiBSi-13.2Cr:PHS r = 3.42 x 10 mg/Nm (R² = 0.9718) 12 Coating NiBSi-13.2Cr:PHS 9 m= 0.154xL + 0.88 9 m= 0.154xL + 0.88 i = 2.89 x 10-2 mg/Nm 6 r -2 ir = 2.89(R² x= 100.9521)mg/Nm

Mass wear, mg wear, Mass 6 (R² = 0.9521) m = 0.1355xL + 0.74 Mass wear, mg wear, Mass 3 3 m = 0.1355xL + 0.74 0 0 0 20406080 Metals 2021, 11, x FOR PEER REVIEW 9 of 19 0 20406080Sliding distance, m

Figure 4. Mass wear vs. sliding distance for NiBSi-13.2CrSliding and NiBSi-13.2Cr: distance, PHS m coatings. Figure 4. Mass wear vs. sliding distance for NiBSi-13.2Cr and NiBSi-13.2Cr: PHS coatings.

Figure 4.12 Mass wear vs. sliding distance for NiBSi-13.2Cr and NiBSi-13.2Cr:-2 PHS coatings. Coating NiBSi-14Cr ir = 2.91 x 10 mg/Nm 10 Coating NiBSi-14Cr:PHS (R² = 0.9747)

8 m = 0.123xL + 0.7 6

-2 4 ir = 2.11 x 10 mg/Nm Mass wear, mg wear, Mass (R² = 0.9724) 2 m = 0.0895xL + 0.56 0 0 20406080 Sliding distance, m

Figure 5. Mass wear vs. sliding distance for NiBSi-14Cr and NiBSi-14Cr: PHS coatings. Figure 5. Mass wear vs. sliding distance for NiBSi-14Cr and NiBSi-14Cr: PHS coatings.

4 -2 Coating NiBSi-16Cr ir = 4.1 x 10 mg/Nm Coating NiBSi-16Cr:PHS (R² = 0.9439) 3 m = 0.0415xL + 0.2 2

Mass wear, mg wear, Mass -2 1 ir = 9.5 x 10 mg/Nm (R² = 0.9453) m = 0.022xL+ 0.2 0 0 20406080 Sliding distance, m

Figure 6. Mass wear vs. sliding distance for NiBSi-16Cr and NiBSi-16Cr: PHS coatings.

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12 -2 Coating NiBSi-14Cr ir = 2.91 x 10 mg/Nm 10 Coating NiBSi-14Cr:PHS (R² = 0.9747)

8 m = 0.123xL + 0.7 6

-2 4 ir = 2.11 x 10 mg/Nm Mass wear, mg wear, Mass (R² = 0.9724) 2 m = 0.0895xL + 0.56 0 0 20406080 Sliding distance, m Metals 2021, 11, 915 9 of 17 Metals 2021, 11, x FOR PEER REVIEW 10 of 19 Figure 5. Mass wear vs. sliding distance for NiBSi-14Cr and NiBSi-14Cr: PHS coatings.

4 12 -2 Coating NiBSi-16Cr i = i4.1r = 4.1 x 10 x-2 10mg/Nmmg/Nm Coating Ni80-20Cr r (R² = 0.9439) 10 Coating NiBSi-16Cr:PHS (R² = 0.9967) 3 Coating Ni80-20Cr:PHS 8 m = 0.0415xL + 0.2 m = 0.1275xL + 0.2 62 -2 ir = 9.5 x 10 mg/Nm

Mass wear, mg wear, Mass -2 4 ir =(R² 9.5 = x 0.9945) 10 mg/Nm Mass wear, mg wear, Mass 1 (R² = 0.9453) 2 mm = =0.1135xL 0.022xL+ - 0.280.2 00 00 20406080 20406080 Metals 2021, 11, x FOR PEER REVIEW 10 of 19 SlidingSliding distance, distance, m m

Figure 6. Mass wear vs. sliding distance for NiBSi-16Cr and NiBSi-16Cr: PHS coatings. Figure 6. Mass wear vs. sliding distance for NiBSi-16Cr and NiBSi-16Cr: PHS coatings. Figure 7. Mass wear vs. sliding distance for Ni80-20Cr and Ni80-20Cr: PHS coatings. 12 -2 Figure 8 representsCoating graphically Ni80-20Cr the dependenceir = 4.1 of xthe 10 wearmg/Nm rate on the concentration of chromium10 for coatings without thermal treatment(R² of the= 0.9967) substrate and for coatings with thermal treatment ofCoating the substrate Ni80-20Cr:PHS for one and the same friction pathway length L = 80 m. The curves8 have non-linear character with a clearly expressed minimum of the wear rate at 16% concentration mof =Cr 0.1275xL for coatings + 0.2 with and without thermal treatment. In the first section,6 in which the chromium concentration is changing within the range from 9.9% to 16% upon increasing of the chromium concentration, the wear rate-2 decreases down to ir = 9.5 x 10 mg/Nm reaching a4 minimal value at chromium concentration 16%, (R²respectively, = 0.9945) for coatings with- Mass wear, mg wear, Mass −2 out thermal treatment of the substrate 𝑖 = 0.91 × 10 mg/Nm and for coatings with ther- −2 mal treatment2 of the substrate 𝑖 = 0.49 × 10 mg/Nm.m = In0.1135xL the second - 0.28 section, at higher chromium concentration (20%) (coatings Ni80-20Cr and Ni80-20Cr:PHS), the wear rate is increasing0 sharply. In spite of the higher chromium concentration the increase in the wear is due to the 0absence of the 20406080 elements B, Si, Cu, and the others, which are contained in the other coatings. These elements in the process of contact interaction of the flame jet with the substrate at the high temperature are forming inter-metallicSliding compounds distance, with m the chromium, which lead to decrease in the wear rate. Figure 7. Mass wear vs. sliding distance for Ni80-20Cr and Ni80-20Cr: PHS coatings. Figure 7. Mass wear vs. sliding distance for Ni80-20Cr and Ni80-20Cr: PHS coatings. 5 Figure 8 represents graphically the dependence of the wear rate on the concentration 4.24 coatings without heat of chromium for coatings without thermal treatmenttreatment of the substrate of the substrate and for coatings with 4 thermal treatment of the substrate for one and thecoatings same friction with heat pathway length L = 80 m. 3.42 The, mg/Nm curves have non-linear character with a clearly expressed minimum of the wear

2 treatment of the substrate rate at 16% concentration3.51 of Cr for coatings with and without thermal treatment. In the 3 2.91 2.89 2.89 first section, in which the chromium concentration is changing within the2.51 range from 9.9% to 16% upon increasing of the chromium concentration, the wear rate decreases down to reaching2 a minimal value at chromium2.11 concentration 16%, respectively, for coatings with- −2 out thermal treatment of the substrate 𝑖 = 0.910.91 × 10 mg/Nm and for coatings with ther- Wear rate х 10 −2 mal treatment1 of the substrate 𝑖 = 0.49 × 10 mg/Nm. In the second section, at higher chromium concentration (20%) (coatings Ni80-20Cr and Ni80-20Cr:PHS), the wear rate is increasing sharply. In spite of the higher chromium0.49 concentration the increase in the wear is due 0to the absence of the elements B, Si, Cu, and the others, which are contained in the other coatings.8 These 10121416182022 elements in the process of contact interaction of the flame jet with the substrate at the high temperature are forming inter-metallicConcentration compounds of Cr, with% the chro-

mium, which lead to decrease in the wear rate. FigureFigure 8. Dependence 8. Dependence of wear of rate wear on rate the onconcentratio the concentrationn of chromium of chromium in the tested in the HVOF-coatings. tested HVOF-coatings. 5 4.24 coatings without heat treatment of the substrate 4 coatings with heat 3.42 , mg/Nm

2 treatment of the substrate 3.51 3 2.91 2.89 2.89 2.51 2 2.11 0.91 Wear rate х 10 1

0.49 0 8 10121416182022 Concentration of Cr, %

Figure 8. Dependence of wear rate on the concentration of chromium in the tested HVOF-coatings.

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MetalsMetals 20212021, 11, 11, x, 915FOR PEER REVIEW 1110 of of 19 17

The curve of the dependence of the wear resistance on the concentration of chromium in coatings without thermal treatment and with thermal treatment of the substrate is re- ciprocalTheThe to curve the curve curve of of the the of dependence wear dependence intensity of of the (Figure the wear wear resistance9). resistance on on the the concentration concentration of of chromium chromium inin coatings coatings without without thermal thermal treatment treatment and and with with thermal thermal treatment treatment of the of substrate the substrate is re- is ciprocalreciprocal2.5 to the to thecurve curve of wear of wear intensity intensity (Figure (Figure 9). 9). without heat treatment 2.5 2.04 of the substrate 2 without heat treatment 2.04

,mN/mg with heat treatment of of the substrate −2 2 the substrate

,mN/mg 1.5 with heat treatment of −2 the substrate 1.1 1.5 1 1.1 1 0.47 0.5 0.35 0.28 0.4 0.47 0.34 Wear resistance x 10 0.5 0.35 0.35 0.280.24 0.29 0.4 0 0.34 Wear resistance x 10 0.29 0.35 0 8 101214161820220.24 Concentration of Cr, % 8 10121416182022 Figure 9. Dependence of wear resistance on the concentrationConcentration of chromium in ofthe Cr, tested % HVOF

coatings. FigureFigure 9. 9.DependenceDependence of of wear wear resistance resistance onon thethe concentrationconcentration of of chromium chromium in thein the tested tested HVOF HVOF coatings . coatings. FigureFigure 10 10 shows shows the the diagram diagram of of the the wear resistanceresistance of of all all the the tested tested coatings, coatings, which which gives gives clear evidence, that the lowest wear resistance is displayed by coatings having the clearFigure evidence, 10 shows that thethe lowestdiagram wear of the resistance wear resistance is displayed of all by the coatings tested havingcoatings, the which lowest lowest concentration of chromium without thermal treatment of the substrate 𝑖 2=0.24 × concentration of chromium without thermal treatment of the substrate ir = 0.24 × 10 Nm/mg, gives2 clear evidence, that the lowest wear resistance is displayed by coatings having the 10while Nm/mg, the while greatest the wear greatest resistance wear resistance is manifested is manifested by the coatings by the coatings having 16% having content 16% of lowest concentration of chromium without thermal treatment of the substrate2 𝑖 =0.24 × content of chromium with thermal treatment of the substrate= 𝐼× = 2.042 × 10 Nm/mg. 10chromium2 Nm/mg, withwhile thermal the greatest treatment wear ofresistance the substrate is manifestedIr 2.04 by 10the Nm/mg.coatings having 16% 2 content of chromium with thermal treatment of the substrate 𝐼 = 2.04 × 10 Nm/mg. 2.5 2.5 2.04 2 Nm/mg 2.04 −2 2

Nm/mg 1.5

−2 1.1 1.5 1 1.1 1 0.47 0.35 0.34 0.35 0.4 0.5 0.24 0.28 0.29 0.47 0.4 0.5 0.28 0.29 0.35 0.34 0.35 Wear resistance x 10 0 0.24 12345678910 Wear resistance x 10 0 12345678910Tested coating

Figure 10. Diagram of the abrasive wear resistance of the tested coatings.Tested coating Figure 10. Diagram of the abrasive wear resistance of the tested coatings. Table4 represents results on the relative wear resistance, calculated by the Formula (4). Figure 10. Diagram of the abrasive wear resistance of the tested coatings. TheTable last two4 represents columns results reﬂect on the the results relative respectively wear resistance, for the inﬂuence calculated of by the the thermal Formula treat- (4).ment The oflast the two substrate columns and reflect the effect the results of chromium respectively concentration for the influence in the powder of the composites. thermal Table 4 represents results on the relative wear resistance, calculated by the Formula treatmentThese results of theare substrate represented and the in theeffect form of ofchromium diagrams concentration in Figures 11 inand the 12 powder. composites.(4). The These last two results columns are represented reflect the inresults the form respectively of diagrams for thein Figures influence 11 ofand the 12. thermal treatment of the substrate and the effect of chromium concentration in the powder com-

posites. These results are represented in the form of diagrams in Figures 11 and 12.

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Metals 2021, 11, x FOR PEER REVIEW 12 of 19

Metals 2021, 11, x FOR PEER REVIEW 12 of 19 Table 4. Relative abrasive wear resistance of the tested coatings.

Table 4. Relative abrasive wear resistance of the tested coatings.Relative Abrasive Wear Resistance Wear Resistance, TableCoating 4. Relative abrasive wear resistance of the testedRelative Inﬂuencecoatings. Abrasive of Heat Wear Resistance Series Sample WearmN/mg Resistance, in the Sliding Inﬂuence of the Designation Treatment of the Coating WearmN/mg DistanceResistance, 80 m InfluenceRelative of HeatAbrasive Wear ResistanceConcentration of Cr, % Series Sample Substrate Influence of the DesignationCoating in themN/mg Sliding InfluenceTreatment of of Heat the Series Sample 2 ConcentrationInfluence of of the Cr, % 1Designation NiBSi-9.9Cr inDistance the 0.24Sliding 80× m10 TreatmentSubstrate of R the1,1 = 1 R1,1 = 1 A Concentration of Cr, % 1 NiBSi-9.9Cr Distance 0.24 × 1080×2 m 2 SubstrateR1,1 = 1 R1,1 = 1 A 2 NiBSi-9.9Cr:PHS 0.28 10 R2,1 = 1.17 R2,2 = 1 2 12 NiBSi-9.9Cr:PHS NiBSi-9.9Cr 0.24 0.28 ×× 10102 2 RR2,11,1 = = 1.17 1 R R1,12,2 == 11 3 NiBSi-13.2Cr 0.29 × 10 R3,3 = 1 R3,1 = 1.21 A 2 B 23 NiBSi-9.9Cr:PHS NiBSi-13.2Cr 0.28 0.29 ×× 10102 R2,1R 3,3= 1.17= 1 R R3,12,2 = = 1.21 1 B 2 4 NiBSi-13.2Cr:PHS 0.35 ×2 10 R4,3 = 1.21 R4,2 = 1.25 34 NiBSi-13.2Cr:PHS NiBSi-13.2Cr 0.29 0.35 ×× 10102 RR4,33,3 = = 1.21 1 R R3,14,2 == 1.211.25 B 2 2 45 5 NiBSi-13.2Cr:PHS NiBSi-14Cr NiBSi-14Cr 0.35 0.340.34 ×× 1010×2 10 R4,3R 5,5= 1.21= 1 R5,5 = 1 R R4,25,1 == 1.251.42 R5,1 = 1.42 CC 22 2 56 6 NiBSi-14Cr:PHS NiBSi-14Cr:PHS NiBSi-14Cr 0.34 0.470.47 ×× 1010× 10 RR6,55,5 = = 1.38 1 R 6,5 = 1.38 R R5,16,2 == 1.421.68 R6,2 = 1.68 C 2 67 NiBSi-14Cr:PHS NiBSi-16Cr 0.47 1.10 ×× 10102 2 R6,5R 7,7= 1.38= 1 R R6,27,1 == 1.684.58 D 7 NiBSi-16Cr 1.10 × 10 R7,7 = 1 R7,1 = 4.58 2 D 8 NiBSi-16Cr:PHS 2.04 × 102 R8,77,7 = 1.84 R7,18,2 = 7.29 7 NiBSi-16Cr 1.10 × 10 2 R = 1 R = 4.58 D 8 NiBSi-16Cr:PHS 2.04 ×2 10 R8,7 = 1.84 R8,2 = 7.29 89 NiBSi-16Cr:PHS Ni80-20Cr 2.04 0.35 ×× 10102 R8,7R 9,9= 1.84= 1 R R8,29,1 == 7.291.46 E ×2 2 109 9 Ni80-20Cr:PHS Ni80-20Cr Ni80-20Cr 0.35 0.400.35 ×× 10102 10 RR10,99,9 = = 1.14 1 R 9,9 = 1 R9,110.2 = = 1.46 1.67 R 9,1 = 1.46 EE 2 2 10 10 Ni80-20Cr:PHS Ni80-20Cr:PHS 0.400.40 × 10× 10 R10,9 = 1.14R10,9 = 1.14 R10.2 = 1.67 R 10.2 = 1.67 2.5 2.5 2 1.84 2 1.84 1.38 1.5 1.17 1.21 1.38 1.14 1.5 1 1.17 1 1 1 1 1.21 1.14 1 1 1 1 1 1 1 0.5

Relative wear resistance 0.5

Relative wear resistance 0 0 12345678910 12345678910Tested coating Tested coating Figure 11. Diagram of the influence of the heat treatment of the substrate on the wear resistance of Figure 11. Diagram of the inﬂuence of the heat treatment of the substrate on the wear resistance of Figurethe tested 11. Diagramcoatings. of the influence of the heat treatment of the substrate on the wear resistance of the tested coatings. the tested coatings. 10 10 8 7.29 8 7.29 6 4.58 6 4 4.58 4 1.68 1.46 1.67 2 111.21 1.25 1.42 1.42 1.68 1.46 1.67 Relative resistance wear 2 111.21 1.25 0 Relative resistance wear 0 12345678910 12345678910Tested coating Tested coating Figure 12. Diagram of the inﬂuence of Cr concentration on the wear resistance of the tested coatings.

Metals 2021, 11, x FOR PEER REVIEW 13 of 19 Metals 2021, 11, 915 12 of 17

Figure 12. Diagram of the influence of Cr concentration on the wear resistance of the tested coatings. The strongest influence on the wear resistance is observed for the thermal treatment of theThe substrate strongest in influence the case ofon thethe coatingwear resistan NiBSi-16Cr:PHS,ce is observed havingfor the thermal concentration treatment of chromium of atthe 16%, substrate for which in the case the wearof the resistancecoating NiBSi-16Cr:PHS, is 1.84 times having higher concentration than that of of the chro- same coating miumwithout at 16%, thermal for which treatment the wear of theresistance support. is 1.84 Next times to it, higher follows than the that coating of the NiBSi-14Cr:PHS same coatingwith concentrationwithout thermal of treatment chromium of the at 14%.support. For Next the remainingto it, follows coatings the coating the NiBSi- influence of the 14Cr:PHSthermal with treatment concentration is almost of chromium one and the at 14%. same For ranging the remaining from 1.14 coatings to 1.21. the influence of Thethe thermal influence treatment of the concentrationis almost one and of the chromium same ranging upon from the abrasive1.14 to 1.21. wear resistance of theThe coatings influence is the of the greatest concentration in the cases of chromi of theum coatings upon the NiBSi-16Cr:PHS abrasive wear resistance and NiBSi-16Cr of (16% theCr). coatings For coating is the greatest with thermal in the cases treatment of the (thecoatings coating NiBSi-16Cr:PHS NiBSi-16Cr:PHS) and NiBSi-16Cr the wear resistance (16%is increased Cr). For coating 7.29 times, with thermal while for treatmen the samet (the coating coating without NiBSi-16Cr:PHS) thermal treatmentthe wear re- (the coating sistance is increased 7.29 times, while for the same coating without thermal treatment (the NiBSi-16Cr) it is increased 4.58 times, which is an extraordinary result. Another good result coating NiBSi-16Cr) it is increased 4.58 times, which is an extraordinary result. Another goodis the result increase is the increase in the wear in the resistancewear resistance of the of coatingsthe coatings NiBSi-16Cr:PHS NiBSi-16Cr:PHS andand Ni80-20Cr:PHS,Ni80- 20Cr:PHS,which becomes which becomes higher higher almost almost to the to same the same degree, degree, respectively respectively 1.68 1.68 and and 1.67 1.67 times. The times.results The onresults the wearon the resistancewear resistance correlate correlate with with the the hardness hardness of of the the coatingscoatings having having different differentconcentrations concentrations of Cr of (Figure Cr (Figure 13). 13).

64 Whithout PHS With PHS 62 62 60 60 60 60 59 59 58 58 57 56 56 55

Hardness, HRC Hardness, 54 52 50 9.9 13.2 14 16 20 Concentration of Cr, %

Figure 13. Diagram of the inﬂuence of Cr concentration on the hardness of the tested coatings. Figure 13. Diagram of the influence of Cr concentration on the hardness of the tested coatings. Metals 2021, 11, x FOR PEER REVIEW 14 of 19 Figure 14 illustrates the diagram of the interconnection between the abrasive wear Figure 14 illustrates the diagram of the interconnection between the abrasive wear resistanceresistance and and the thehardness hardness of the of tested the tested coatings. coatings.

Figure 14. Diagram of abrasive wear resistance and hardness of the tested coatings. Figure 14. Diagram of abrasive wear resistance and hardness of the tested coatings.

5. Regression Models

The experimental curve of the dependence of the wear intensity 𝑖 on the chromium concentration in the range 9.9% ≤ 𝑤 ≤ 16% is considered (Figure 14). The section of the curve where 𝑤 > 16% is not considered because it includes coatings No 9 and No 10 (Ni80-20Cr and Ni80-20Cr:PHS) with different chemical composition from the other coatings. These coatings contain only nickel (80%) and chromium (20%), which does not give us reason to analyze them in parallel with the other coatings. Experimental results for the wear intensity at more points on the curve in Figure 8 are presented in Table 5. Graphically, the dependence of the wear intensity on the percentage of chromium is shown in Figure 15.

Table 5. Wear rate at different chromium concentration, %.

w, Сoncentration of Cr, (%) 9.9 11.8 13.2 14.0 15.2 16.0 𝑖 , wear rate, mg/Nm, without heat 4.24 3.82 3.42 2.91 2.1 0.91 treatment of the substrate 𝑖 , wear rate, mg/Nm, with heat 3.51 3.35 2.89 2.11 1.35 0.49 treatment of the substrate

Metals 2021, 11, 915 13 of 17

5. Regression Models

The experimental curve of the dependence of the wear intensity ir on the chromium concentration in the range 9.9% ≤ w ≤ 16% is considered (Figure 14). The section of the curve where w > 16% is not considered because it includes coatings No 9 and No 10 (Ni80-20Cr and Ni80-20Cr:PHS) with different chemical composition from the other coatings. These coatings contain only nickel (80%) and chromium (20%), which does not give us reason to analyze them in parallel with the other coatings. Experimental results for the wear intensity at more points on the curve in Figure8 are presented in Table5. Graphically, the dependence of the wear intensity on the percentage of chromium is shown in Figure 15.

Table 5. Wear rate at different chromium concentration, %.

w, Concentration of Cr, (%) 9.9 11.8 13.2 14.0 15.2 16.0 i , wear rate, mg/Nm, without r 4.24 3.82 3.42 2.91 2.1 0.91 heat treatment of the substrate i , wear rate, mg/Nm, with heat Metals 2021, 11, x FOR PEER REVIEW r 3.51 3.35 2.89 2.11 1.3515 of 0.49 19 treatment of the substrate

5 without heat treatment of 4.24 the substrate 3.82 4 with heat treatment of the 3.42 substrate

, mg/Nm 3 2.91 −2 3.51 3.35 2.89 2 2.1 2.11

1 1.35 0.92 Wear rate x 10 0.49 0 9 101112131415161718 Concentration of Cr, %

FigureFigure 15. Dependence 15. Dependence of wear of wear rate rateon chromium on chromium concentration concentration range range from from 9.9% 9.9% to 16%. to 16%.

BasedBased on the on regression the regression analysis, analysis, analytical analytical dependences dependences of the wear of the rate wear on the rate chro- on the miumchromium concentration concentration were obtained, were obtained, presente presentedd as second- as second- and third-degree and third-degree polynomials. polynomials. For coatings without heat treatment of the substrate the dependence has the follow- For coatings without heat treatment of the substrate the dependence has the following form ing form 3 2 ir(w) = a3w + a2w (5) 𝑖 𝑤 =𝑎 𝑤 +𝑎 𝑤 (5) The results are presented on the next Figure 16 . TheThe results Adjusted are presented R Square on is the 0.745894827 next Figure and 16. shows that 74.58% of the variance of the intensity of wear is predictable from chosen factors (w3, w2) which are adequately included in the model.i , wear The rate, value mg/Nm, of the significancewithout heatF with treatment significance of the level 0.05 is 0.00012 < 0.05 (0.012%r < 5%), i.e., the results are reliable (statistically significant) and the model is adequate. p-values of the coefficients of thesubstrate regression equations with level of significance 0.05 are smaller5.00 than 0.000032, i.e., they are smaller than 0.05, which means that the coefficients are measured statistically significant, and the adequacy of the model is confirmed. 4.00 calculated , mg/Nm −2 3.00

2.00

1.00 wear rate x 10 rate wear 0.00 9.00 11.00 13.00 15.00 17.00 w, Сoncentration of Cr, (%)

Figure 16. Analytical dependences of the wear rate on the chromium concentration for coatings without heat treatment, 𝑖𝑤 = −0.00599819𝑤 + 0.099501902𝑤 .

The Adjusted R Square is 0.745894827 and shows that 74.58% of the variance of the intensity of wear is predictable from chosen factors (w3, w2) which are adequately included in the model. The value of the significance F with significance level 0.05 is 0.00012 < 0.05 (0.012% < 5%), i.e. the results are reliable (statistically significant) and the model is adequate. p-values of the coefficients of the regression equations with level of significance 0.05 are smaller than 0.000032, i.e., they are smaller than 0.05, which means that the coefficients are statistically significant, and the adequacy of the model is confirmed.

Metals 2021, 11, x FOR PEER REVIEW 15 of 19

5 without heat treatment of 4.24 the substrate 3.82 4 with heat treatment of the 3.42 substrate

, mg/Nm 3 2.91 −2 3.51 3.35 2.89 2 2.1 2.11

1 1.35 0.92 Wear rate x 10 0.49 0 9 101112131415161718 Concentration of Cr, %

Figure 15. Dependence of wear rate on chromium concentration range from 9.9% to 16%.

Based on the regression analysis, analytical dependences of the wear rate on the chromium concentration were obtained, presented as second- and third-degree polynomials. For coatings without heat treatment of the substrate the dependence has the following form

Metals 2021, 11, 915 14 of 17 𝑖𝑤 =𝑎𝑤 +𝑎𝑤 (5) The results are presented on the next Figure 16.

ir, wear rate, mg/Nm, without heat treatment of the substrate 5.00 measured 4.00 calculated , mg/Nm −2 3.00

2.00

1.00 wear rate x 10 rate wear 0.00 9.00 11.00 13.00 15.00 17.00 Metals 2021, 11, x FOR PEER REVIEW w, Сoncentration of Cr, (%) 16 of 19

FigureFigure 16. AnalyticalAnalytical dependences dependences of ofthe the wear wear rate rateon the on chromium the chromium concentration concentration for coatings for coatings 𝑖 𝑤 = −0.00599819𝑤 + 0.099501902𝑤 without heat treatment, ( ) = − 3 + 2 . withoutFor coatings heat treatment, withoutir w heat treatment0.00599819 wof th0.099501902e substratew the. dependence has the following form:ForThe coatingsAdjusted without R Square heat is 0.745894827 treatment ofand the shows substrate that 74.58% the dependence of the variance has the of follow-the intensity of wear is predictable from chosen factors (w3, w2) which are adequately included ing form: in the model. The value of the significance𝑖 𝑤 =𝑏 F with𝑤3 significance+𝑏2𝑤 level 0.05 is 0.00012 < 0.05 (6) ir(w) = b3w + b2w (6) (0.012%The results < 5%), are i.e. presentedthe results areon thereliable Figure (statistically 17. significant) and the model is adequate.The p-values results of are the presented coefficients on theof the Figure regressi 17. on equations with level of significance 0.05 are smaller than 0.000032, i.e., they are smaller than 0.05, which means that the coefficients are statistically significant, and the adequacy of the model is confirmed. ir, wear rate, mg/Nm, with heat treatment of the substrate 5.00 measured

4.00 calculated , mg/Nm 2 − 3.00

2.00

1.00 wear rate x 10 0.00 9.00 11.00 13.00 15.00 17.00 w, Сoncentration of Cr, (%)

FigureFigure 17. 17. AnalyticalAnalytical dependences dependences of of the the wear wear rate onon thethe chromiumchromium concentration concentration for for coatings coatings with 3 2 heat treatment, ir(w) = −0.005371987w + 0.087400503 w . with heat treatment, 𝑖𝑤 = −0.005371987𝑤 + 0.087400503𝑤 . The Adjusted R Square is 0.74616892 and shows that 74.62% of the variance of the The Adjusted R Square is 0.74616892 and shows that 74.62% of the variance of the intensity of wear is predictable from chosen factors (w3, w2), i.e., they are adequately 3 2 intensityincluded of inwear the is model. predictable The value from ofchosen the significance factors (w , F w with), i.e., significance they are adequately level 0.05 isin- cluded0.00011 in the < 0.05 model. (0.011% The < value 5%), i.e.,of the the significance results are reliable F with(statistically significance significant) level 0.05 is and 0.00011 the < 0.05model (0.011% is adequate. < 5%), i.e., P-values the results of the are coefficients reliable (statistically of the regression significant) equations and with the levelmodel of is adequate.significance Р-values 0.05 areof the smaller coefficients than 0.000019, of the regressi i.e., theyon are equations smaller with than 0.05,level whichof significance means 0.05that are the smaller coefficients than are0.000019, statistically i.e. they significant, are smaller and thethan adequacy 0.05, which of the means model that is confirmed. the coefficients are statistically significant, and the adequacy of the model is confirmed.

Metals 2021, 11, 915 15 of 17

6. Conclusions The present research work represents comparison of results for the characteristics of the wear process and the wear resistance of composite powder coatings, deposited by means of supersonic flame jet (HVOF), which contain composite mixtures Ni-Cr-B-Si having different concentrations of chromium—9.9%, 13.2%, 14%, 16%, and 20%—at one and the same size of the particles (45 µm) and equal content of the other elements, boron and silicon. The coating, prepared to have 20% Cr, does not contain the elements B and Si. Each powder composition was applied to obtain coating without preliminary thermal treatment of the substrate and with preliminary thermal treatment of the substrate up to 650 ◦C. The coatings have been tested under identical regimes of dry friction along the surface with firmly attached abrasive particles using a tribo-tester “Pin-disc”. Results have been obtained on the dependence of the mass wear as a function of the length of the pathway of friction, the variation of the wear rate depending on the concentration of chromium for coatings without thermal treatment of the substrate and with thermal treatment of the substrate. It has been ascertained that for all coatings the preliminary thermal treatment of the substrate leads to a decrease in the wear rate. It has been shown that upon increasing the concentration of chromium the wear rate is decreasing non-linearly, whereupon it reaches minimal values at 16% Cr. In the case of coatings having 20% concentration of Cr the wear rate is higher, which is due to the absence of the components B and Si in the composite mixture. In this case no new inter-metallic structures are formed, having high hardness and high wear resistance. A diagram of the interconnection between the hardness of the coatings and their abrasive wear resistance is represented. The destruction of the surface of the coating due to abrasive wear is mechanical in nature. The main mechanisms of abrasive wear are two-plastic deformation and micro- cutting. At a low concentration of chromium in the coating, its hardness is lower, the abrasive particles penetrate into the coating, and initially, part of the material is pushed out, forming scratches and grooves on the surface of the coating. Due to the repeated cyclic interaction, the material separates from the surface and the intensity of wear increases as a result of the formation of submicro- and micro-cracks from fatigue. With a higher chromium content in the coating, the hardness of the coating is higher than the presence of chromium intermetalloid compounds. In this case, the abrasive wear takes place by the mechanism of micro-cutting of the surface layer of the coating, in which very thin and shallow scratches are observed and the wear rate is significantly lower. Based on the regression analysis, analytical dependences of the wear rate on the chromium concentration were obtained for coatings without heat treatment and coatings with heat treatment of the substrate, presented as polynomials of second and third degree.

Author Contributions: Conceptualization, Z.K. and M.K.; methodology, M.K.; software, Y.S.; valida- tion, Z.K., M.K. and Y.S.; formal analysis, Z.K.; investigation, M.K.; resources, Z.K.; data curation, Y.S.; writing—original draft preparation, M.K.; writing—review and editing, Z.K.; visualization, M.K.; supervision, M.K.; project administration, M.K.; funding acquisition, Z.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. Metals 2021, 11, 915 16 of 17

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