Influence of Substrate Type Made of WC-Co on CrN/CrAlN Coatings’ Durability During Machining of Particleboard

Abstract

This paper investigates the influence of substrate grain size on the behavior of a multilayer CrN/CrAlN coating, with the bilayer thickness varying across the cross-section in the range of 200–1000 nm. The substrate tools were made of WC-Co sintered carbide with three different grain sizes. The coatings were subjected to mechanical and tribological tests to assess their performance, including nanohardness, scratch resistance, and tribological testing. The coating’s roughness was measured using a 2D profilometer. Additionally, the chemical composition and surface morphology were analyzed using Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX). The durability tests were performed on an industrial CNC machine tool on the particleboard. The results revealed that tools with ultra-fine nano-grain (S) and micro-grain (T) WC-Co substrates exhibited a significant increase in tool durability by 28% and 44%, respectively. Significant differences in the microgeometry of the substrate U, especially in relation to the tool based on substrate S, explain the lack of improvement in its durability despite the use of a multilayer coating.

1. Introduction

The literature review shows that tool coatings containing chromium compounds are often recommended for wood processing. Coatings of this type are produced mainly on cemented carbide and steel substrates. The tools studied in this work are made of cemented carbide. Many papers have been published that focus on optimizing the application process [1,2,3]. In the case of chromium compounds, one can often find studies in which authors such as [3,4] analyze the effect of the crystallographic form on the physico-mechanical properties of tool coatings. The aim of these studies is, among others, to determine the optimal quantitative proportions between the hexagonal form of Cr2N and the cubic form of CrN. According to [5], this is determined by the nitrogen content in the gas mixture used during the application process. In the studies above, this share ranged from 10%, where mainly Cr2N was observed, to 20%, where the CrN phase was the dominant phase. The microhardness and scratch resistance tests indicate that Cr2N is harder and has better resistance to dynamic loads but is characterized by worse adhesion.

Nevertheless, both variants allow for a significant reduction in the friction coefficient during the processing of wood materials. In addition, the coating protects the tool against cobalt loss, thus preventing the tearing out of carbide grains. By increasing the nitrogen content to 30%, it is possible to obtain only the cubic form of CrN, as shown by [4,6]. The hardness of the coating in which both phases occur (16.95 GPa) significantly exceeds that of the coating in which only CrN occurs (10.50 GPa). Other factors influencing the microstructure of the coating include the current frequency in the DC Magnetron Sputtering deposition process (110, 160, and 280 kHz) analyzed by [7]. The increase in this value also has a positive effect on the nanohardness of the coating, which reached a level of about 20 GPa. In addition to the frequency, the current voltage applied to the electrodes is also essential. Using XRD analysis, [8] observed significant changes in the crystallographic lattice, as a result of which the Cr/N ratio increased from 1.4 to 2.0, respectively, when changing the substrate bias from 60 to 120 V.

Durability tests on the Cr2N coating during veneer cutting with HSS knives were conducted by [9]. The surface of the tools was initially nitrided and then covered with a CrN sublayer. The modified tool showed high resistance to damage caused by dynamic loads caused by mineral inclusions. Thanks to the use of the coating, the shape deformations of the knives were smaller, resulting in better processing quality. Ref. [10] also analyzed both crystallographic varieties of chromium compounds in terms of durability. Scratch tests showed relatively high adhesion of both single CrN and Cr2N and multilayer Cr2N/CrN coatings. It turned out that the abrasion resistance of the multilayer coating was 2 times higher than that of the CrN coating and even 20 times higher than that of the Cr2N coating. In addition, a 3× improvement in the roughness of the machined surface was achieved, and no significant deterioration was observed with the increase in the cutting path during the processing of wood materials. Further efforts to obtain even better functional properties of coatings based on chromium compounds primarily enhance their hardness and high-temperature oxidation resistance. The aluminium addition used by [11] provided a compact microstructure with fewer defects. Nanohardness increased in the case of CrAlN coating in relation to CrN from 18 to 33 GPa. In addition, oxidation resistance changes favorably from 600 to 800 °C. Although the roughness of both coatings increases with the annealing time, this increase is smaller in the case of CrAlN. According to [12], the multilayer CrN/AlN coating is characterized by smaller grains than each of them separately. The optimal thickness of the subcoating (bilayer) consisting of CrN and AlN was considered 60 nm. With their total number n = 60, the hardness and modulus of elasticity were 28 GPa and 280 GPa, respectively. Ref. [13] also showed changes in the CrN/CrAlN multilayer coatings in hardness with increasing annealing temperature at levels acceptable for their use up to 600 °C. In the multilayer coatings of the CrN/CrAlN type, the proportions between the Cr and Al content play an important role. The results of the research conducted by [6] show that the reduction of the CrN/CrAlN sublayer thickness by changing the Al/(Cr + Al) ratio resulted in an increase in the coating hardness to 42 GPa, and a decrease in the friction coefficient and the wear coefficient in the wear tests. The optimum proportion between Al and Al + Cr was considered to be 59.1%.

Durability tests of coatings containing Cr during the milling of wood materials were conducted by [14]. With a nitrogen content in the plasma of 20%, a working pressure of 4 bar and a voltage applied to the target of 900 V, the hardness of the coating applied to tools made of sintered carbide reached approx. 20 GPa. The increase in tool durability during the milling of the MDF board was 2.5×. In the work above, the authors verified the influence of the substrate microstructure on the behavior of the CrN-type nano-coating. A higher share of nitrogen in the plasma also brings positive effects in this respect. In previous work by the authors of this article, the influence of different types of cemented carbide substrates on the properties of the same type of TiN/AlTiN and TiAlN/a-C:N coatings was also observed [15].

The studies conducted so far indicate a very high potential of multilayer CrN/CrAlN-type coatings. Their structure should be further optimized by using gradient coatings of different thicknesses of bilayers, where the properties of individual sublayers change on the cross-section, thus enabling even better functional properties. One of the key factors determining the durability of tool coatings is the microgeometry of the blade [16]. Many researchers deal with this issue, indicating that the optimal shape of the blade rounding, thanks to eliminating excessive and undesirable internal stresses in the coating, has a decisive influence on its durability. Changing the geometric parameters of the blade results, among others, improves tool stability [17,18,19,20]. Ref. [21] emphasize that shifting the rounding tip, i.e., the so-called point n towards the rake face, on which the chip formed during cutting is deposited, causes a significant increase in the thermo-mechanical loads on the blade in this area. As a result, an accelerated wear process can be observed on the rake face side and a tendency to undergo chipping on the edge. In the opposite situation, we are dealing with a more intensive abrasion process on the flank side. There are few literature reports on the effect of microgeometry on the durability of tools coated with coatings intended for wood and other wood materials. This topic has not been thoroughly analyzed, and one of the few scientific papers, if not the only one, concerns the effect of changing the radius of the cutting edge rounding and thus the interference of the microgeometry of the blade with different types of coatings containing chromium compounds intended for cutting, applied to inserts made of WC-Co sintered carbide [22]. In this paper, the influence of the grain size of the WC-Co cemented carbide substrate on the characteristics, and in particular the microgeometry, of the multilayer CrN/CrAlN coating is investigated.

2. Materials and Methods

2.1. Materials

Three types of WC-Co sintered carbides blades with different tungsten carbide grain sizes and cobalt contents were used in the study (S, U, and T). Based on the literature, the various properties of S, U, and T substrates may be more influenced by the substrate’s grain size than by the cobalt content [23]. This is due to the fact that the Co content does not vary over a wide range. The interchangeable cutting inserts manufactured by FABA S.A (Baboszewo, Poland)had dimensions of 30 mm × 12 mm × 1.5 mm. The technical data of the tool substrate material as provided by the manufacturer are given in

Table 1. The technical data of the tool substrate material.

Tool Index	Hardness HV10	Grain Description	Binder [%]	WC Grain Size [µm]
S	2250	Ultra-fine nano	2	0.2–0.5
U	1920	Submicron	4.2	0.5–0.8
T	1800	Micro	6	0.7–1

.

The wood material on which the blunting process was carried out was a three-layer standard chipboard with a thickness of 18 mm. The physical and mechanical properties of the processed material are shown in

Table 2. Mechanical and physical properties of machined particleboard.

Property	Value
Density [kg/m3]	650
Flexural strength [N/mm2]	13.1
Elastic modulus [N/mm2]	3200
Strength in pull out of screws test [N/mm]	70.9
Hardness in Brinell scale [HB]	2.61
Mineral contamination [%]	0.18
Tensile strength [N/mm2]	0.37

[24,25,26,27,28,29].

2.2. Coatings Synthesis

The CrN and multilayer coatings were deposited on tungsten carbide substrates HW described earlier by using an industrial standard DC magnetron sputtering system (Kenosistec model KS40V-113K12). Rectangular targets (406.4 mm × 127 mm × 6.35 mm) of Cr (purity of 99.95%) and of Al (purity of 99.5%) were placed side by side in the deposition chamber. The substrate holder is a planetary double rotating carousel. The distance between the targets and the rotating substrate holder is 120 mm. An Ar + N₂ gas mixture (purity 99.99%) was used during the deposition process. The residual pressure of the chamber was 8 × 10⁻⁴ Pa. The substrates were cleaned ex situ (ultrasonic bath in acetone and ethanol for 10 min, respectively) and in situ (under 150 sccm Ar etching at a bias voltage of −700 V, a pressure of 0.7 Pa during 5 min). The targets were also cleaned in situ under a 100 sccm Ar flow rate, at 0.5 Pa, with a target bias voltage of −340 V during 10 min. The rotation speed of the substrate holder was fixed to 1.5 rpm. The working pressure was fixed to 0.5 Pa.

2.3. Architecture and Chemical Composition

The coatings’ architecture design is depicted in Figure 1. Conditions in the chamber during coating application are listed in

Table 3. Deposition conditions of the coatings.

Argon flow (sccm)	Cr	92
	CrN	68.6
	CrAlN	68.6
Nitrogen flow (sccm)		33.3
Working pressure (Pa)		0.5
Power applied on the chromium target (W)	Cr	1500
	CrN	1500
	CrAlN	1500
Power applied on the aluminium target (W)		1000
Substrate rotation speed (rpm)		1.5
Deposition time (min)	Cr	10
	CrN	60/30/15/12 *
	CrAlN	60/30/15/12 *
Deposition temperature (°C)		300
Substrate bias voltage (V)	Cr	−500
	CrN	−500
	CrAlN	−500

* The deposition time presented for CrN/CrAlN coating is the deposition time for each monolayer from the thicker to the thinner.

. The Cr underlayer is present in each coating and the thickness is about 200 nm, adapted to the substrate roughness [30]. The thickness of the Cr/CrN/CrAlN multilayer is 2 µm and is made of 4 pairs of CrN/CrAlN with decreasing thickness from the substrate interface to the top surface. The thickness of each CrN/CrAlN pair is 1000 nm, 500 nm, 250 nm, and 200 nm, respectively. It is a repetition of the work of [31] to verify the influence of the chemical composition (CrAlN instead of AlCrN). Information on the physico-mechanical and chemical properties of the deposited coating can be found in the work of [32]. However, it should be emphasized that the design of the layer architecture does not reflect their actual arrangement.

2.4. Coatings Characterization

The Noran energy-dispersive X-ray spectroscopy (EDS) microanalysis system installed in a SEM (Scanning Electron Microscope; SU-70, HITACHI, Tokyo, Japan) made it possible to determine the coatings’ surface topography and chemical composition.

Surface roughness measurements were carried out on the sample surfaces using a SURFTEST SJ-210 (Mitutoyo, Kawasaki, Japan) contact profilometer over an 8 mm section. Six average roughness (Ra) measurements were taken. The following parameters were adopted as indicators of surface quality, namely average roughness (Ra), root mean square (Rq), and maximum roughness (maximum height of profile, Rz).

The thickness tests were carried out using a Recalo 2 ball tester. On each sample, abrasion was performed with a 30 mm diameter (52100 AISI) steel ball, which allowed for the assessment of the coating thickness and the qualitative evaluation of its adhesion at the location where the spherical indentation was made.

Coatings’ adhesion to the substrates was evaluated using a CSM Instruments RST scratch tester (CSM Instruments, Peseux, Switzerland) equipped with a diamond stylus featuring a 200 µm spherical tip. The applied load was progressively increased from 1 N to 50 N along a scratch length of 5 mm. The adhesion of the coatings was evaluated on the basis of acoustic emission measurements and post-test scratch images based on one of the three measurements taken. The critical load LC2 was determined based on the points where the coating starts to fail. LC2 refers to the load at which significant failure or damage to the material or coating occurs, such as cracking or chipping.

2.5. X-Ray Diffraction

The phase composition of the as-deposited layer was determined on an X-ray diffractometer—Anton Paar XRDynamic 500 (Anton Paar GmbH, Graz, Austria) (Cu lamp, parallel beam, scanning step 0.04°, 25–100° 2θ range). To reduce penetration depth of X-rays and as a result of the increase signal from the near-surface layer, the grazing incidence measurement mode with an incidence angle at 1° was applied. The ICDD PDF 4+ Axiom 2024 database was used for the analyses.

2.6. Nanohardness Studies

Nanoindentation tests were performed using a NanoTest Vantage Alpha device from Micromaterials Ltd. (Wrexham, Wales, UK) with a Berkovich indenter. The measurements were carried out under a 0.5 mN load, at which the indenter penetration depth did not exceed 10% of the coating thickness. A 10 s load increase time, a 10 s load decrease time, and a 5 s Dwell Period were applied.

During the measurements, the displacement of the indenter as a function of the applied load was recorded, allowing for the determination of load-displacement curves. After each measurement, thermal drift analysis of the material was conducted for 5 s at a final load not exceeding 10% of the nominal measurement load. A total of 25 measurements were performed on each sample in a 5 × 5 grid with 20 μm spacing.

The manufacturer’s software was used to correct the tip compression effect and instrument drift. Data analysis was performed using the Oliver–Pharr method to determine the nanohardness and reduced Young’s modulus, taking into account the current indenter geometry resulting from operational wear.

2.7. Tribological Tests

An AC motor rotating at 1400 rpm, with an output shaft featuring a cone for mounting a ball with a diameter of 4.762 mm made of ZrO₂ material, guided vertically, applied a force of 3.83 N between the ball and the tested surface of the plate.

The ball rotated around its axis while the sample remained stationary. Seven repetitions were performed at different locations on the application surface. The vertical displacement of the motor was recorded by an inductive sensor with a resolution of 0.1 µm. The inductive sensor was connected via a cable from IBR Messtechnik to a computer that recorded the displacement of the motor as a function of time.

2.8. Microgeometry Studies

In this study, due to the significant difference in the durability of the coating applied to the U-type substrate compared to others (S and T), despite identical application conditions, an analysis of the blade’s microgeometry was conducted to explain this phenomenon. For this purpose, the procedures and methodology outlined in detail in the work by [33] were used. Figure 2 and Figure 3 show the basic wear indicators and geometric parameters analyzed in this study.

All analyzed wear indicators and geometric parameters showed in

Table 4. Summary of blade wear indicators and other geometric parameters and their classification.

Type of Indicator	Group 1	Group 2	Group 3
Indicator symbol	Sα, Sy	Aα, Aγ	PAα
	Rα, Rγ	Dα, Dγ	NP_ Aα
	rα, rγ	np, nq	hn, h
		apex n	γeff
		Δr	Lα, Lγ

, describing the blade geometry were divided into 3 groups. Group 1, shown in the table above, includes the most frequently used tool wear indicators. Group 2 contains coefficients describing the asymmetry of rounding, assuming that the bisector of the sharp angle will be vertical, Figure 1. Group 3 includes geometric parameters determined by taking into account the position of the blade in the working position, i.e., maintaining the appropriate angular values such as, among others, cutting angle δ (angle α + angle β), Figure 2.

2.9. Durability Tests

A Busellato machining center JET 130 was used during this research. Assuming that the feed per tooth Δz will be 0.15 mm, the following values were adopted: spindle speed 18 mm RPM and feed speed 2.7 m/min. The cutting diameter of the head produced by the company FABA S.A. equipped with one replaceable blade was 40 mm. For each variant, 4 blades were used (overall, 8 cutting edges). During statistical processing of the data, the extreme values, i.e., the minimum and maximum value for a given variant, were considered unrepresentative and rejected. In order to avoid problems related to uneven blunting of individual knives, a single-blade head was used. The aforementioned issue results, among others, from the inaccuracy of the tool mounting system used by the head manufacturer.

The feed distance per one dulling cycle was 0.7 m. Then, after each cycle, the tool was subjected to wear measurement on a workshop microscope until the dulling criterion VB max equal to 0.2 mm was reached. The VB_max geometric wear index of the blade takes into account both the loss of the cutting edge in relation to its original profile and the extent of the zone where the friction effects between the material being processed and the blade application surface are visible (Figure 4). The details of blunting process with view on milling head is presented in Figure 5. In order to determine the cutting path traveled by the blade based on the feed path, the following formula was used:

$$L_t={\displaystyle \frac{V_C\frac{L}{V_T}}{2}}={\displaystyle \frac{\pi Dn}{2}}·{\displaystyle \frac{L}{V_T}}$$

where:

D—diameter of tool [m]

n—rotational spindle speed [1/min]

Vt—feed speed [m/min]

Vc—cutting speed [m/min]

L—feed distance [m].

Figure 4. Scheme of VB _max tool wear indicator measurement.

Figure 4. Scheme of VB _max tool wear indicator measurement.

Figure 5. View of the milling head with knife and scheme of blunting process.

Figure 5. View of the milling head with knife and scheme of blunting process.

3. Results and Discussion

3.1. Microstructure, Chemical Composition, Roughness, and Thickness of the Coatings

The photos taken with SEM (Figure 6a–c) showed clear differences in the morphology of the coating surface depending on the substrate structure, including direction of grinding during the process in their manufacturing and preparation for work. In addition, the coatings applied to the U-type (submicron) cutting knives show a discontinuous structure with visible grooves and craters at the bottom of which the substrate is visible. Craters and grooves are caused by the preparation of the substrate, which was subjected to the same pretreatment. Each type of substrate exhibits deeper or smaller grooves depending on its hardness. This type of defect may be dangerous due to the risk of coating delamination during machining, thus leading to the so-called Catastrophic Blade Failure (CBF) in extreme cases. This condition of the tool requires its withdrawal from further use. On S (ultra-fine nano) and T (micro) substrates, only single blisters are visible; however, in both cases, the coating seems to be more homogeneous and has a relatively uniform thickness.

On the basis of the EDX analysis carried out at 5 kV, both the mass and the atomic share of individual elements in the coating were determined, given in Table 3. The summary given in

Table 5. The chemical composition of the coatings.

	N	Al	Cr
Mass %
Tool S	22.2 ± 0.5	5.9 ± 0.5	71.9 ± 1
Tool U	21.9 ± 0.6	5.7 ± 0.1	72.5 ± 0.6
Tool T	22.2 ± 0.3	5.9 ± 0.2	72.0 ± 0.3
Atomic %
Tool S	49.7 ± 0.7	6.9 ± 0.5	43.5 ± 1.1
Tool U	49.3 ± 0.8	6.7 ± 0.1	44.0 ± 0.8
Tool T	49.7 ± 0.5	6.8 ± 0.3	43.4 ± 0.4

indicates that the chemical composition of the coating is practically almost identical regardless of the type of substrate. The atomic fraction of Al is in the range of 6.3–6.9%. The primary component of the coating is intended to be chromium with an atomic fraction of around 45%.

The surface roughness of CrN/CrAlN coatings deposited on S, U, and T substrates is shown in Figure 7. No influence of the substrate on the roughness parameters of the coatings was observed. Since the roughness of the tools was measured along the grooves, no significant differences in roughness were found, which is not exactly consistent with the coating topography observed in the SEM images (Figure 6a–c).

Further studies focused on determining and verifying the thickness of the applied coatings. Observation of the craters formed during the ball test revealed a high degree of surface inhomogeneity. The surface is characterized by numerous scratches after grinding, and jagged edges at the transition from the sample surface to the crater formed due to the abrasion process. Abrasions were performed on the sample marked as T, which revealed a coating with a thickness of 1–1.09 µm. The resulting spherical microsections show that the coating fills the unevenness of the substrate, which may indicate its good adhesion. The average measurement from all abrasions was 1.25 µm ± 0.23 µm

3.2. XRD Results

Figure 8 shows the diffraction pattern of obtained layers. Only a cubic phase of the CrN type—space group Fm-3m (ICDD file no. 04-002-0406)—was observed in the coating. Part of the signal comes from the substrate—tungsten carbide—space group P-6m2 (ICDD file no. 04-004-5823). It should be noted that in the case of samples U and T, the intensity of the (200) peak is stronger than the intensity of (111), while in the S sample, it is the opposite case. This means that a different morphology of the coating structure and properties should be expected, especially in the case of sample S. In the paper [34], it was observed that the variable intensity of peaks (111) and (200) resulted from different nitrogen concentrations in the PVD process, but the above reason is unlikely, due to the production of coatings on all substrates in one batch. However, correlating with the chemical composition of the substrate, it can be said that with decreasing cobalt concentration in the cemented carbide substrate, peak (111) has a higher intensity, where in the case of sample S, it is the highest.

3.3. Thickness Studies

On the coating deposited on the S substrate, several abrasions were performed, revealing coatings with good adhesion and a 1.04–1.22 µm thickness (Figure 9a). The average of all abrasions was 1.16 µm ± 0.09 µm. Spherical microsections on the U sample revealed a coating thickness of 1.13–1.18 µm. The average of all measurements was 1.16 µm ± 0.02 µm. Microscopic observation of the samples reveals remnants of the PVD process (solidified particles and craters from fallen particles—Figure 9b).

3.4. Scratch Tests Results

In order to analyze the adhesion of the CrN/CrAlN coatings to the S, U, and T substrates, a scratch test was performed, and critical load 2 (LC2) was evaluated (Figure 10). It is observed that in the case of the coating deposited on the S-type substrate, the values of LC2 were the lowest, but at the same time, the differences between each sample were not substantial.

3.5. Nanohardness Results

During nanoindentation measurements at a constant load of 0.5 mN, the T coating exhibited the highest hardness (10.42 GPa) with an indentation depth of 33.1 nm ± 5.8 nm (Figure 11). The other coatings, labeled S and U, showed similar hardness values (7.86 GPa and 7.80 GPa, respectively) and comparable indentation depths (41.4 nm ± 11.6 nm and 39.4 nm ± 6.0 nm, respectively).

The T coating also had the highest reduced Young’s modulus (Er), reaching approximately 228.6 GPa, while the S and U coatings exhibited lower values (196.5 GPa for S and 185.3 GPa for U).

3.6. Durability Results

The results presented in Figure 12 show a statistically significant improvement in the durability of the blades based on the ultra-fine nano-grain described as S and the coarsest (micro) grain designated as T. To increase the clarity of the results, the colors of the bars in the figure have been differentiated. In the case of blades designated as S, this increase was 43%. On the other hand, the modified T-type blades showed an increase in durability of up to 53%.

The U-type blades (submicron) behaved completely differently. In their case, the application of the multilayer CrN/CrAlN coating did not bring any improvement in their performance properties. The reasons for this state of affairs can be found in the defects of the surface morphology shown earlier in the SEM images.

3.7. Tribological Test Results

Figure 13a, in comparison with Figure 13b, indicates a significant effect of the coating on reducing wear. The wear of the coatings is an order of magnitude lower than that of the substrate alone. Figure 13a confirms that the lowest wear occurs on the coating applied to the S-type substrate. In contrast, the wear of the coating applied to the U-type substrate is the highest, which is consistent with the results obtained in the durability tests during the processing of the particleboard. However, these differences are not as significant as in the durability tests, as the processing was not as severe.

3.8. Microgeometry Results

Due to the fact that the surface morphology studies and nanohardness analysis did not clearly explain the source of problems related to the durability of the coating applied to substrate U, detailed studies of the microgeometry of the rounding were carried out. Therefore, the preliminary procedure before the appropriate analysis of variance (ANOVA) was carried out. The first step was to verify whether the variances in the individual variable groups (S, U, T) differ statistically significantly, i.e., whether they are homogeneous (Loeven’s test). The results of the analysis conducted using Statistica software (TIBCO Software Inc, version 13.00.0) showed that they are heterogeneous for the indicators considered. Therefore, the proper ANOVA analysis was abandoned. To obtain information about which variant was responsible for rejecting the null hypothesis of homogeneity of variance, post hoc tests, also known as multiple-comparison tests, were used. The NIR test was chosen.

The first group of analyzed variables included the wear indicators S_α,γ, R_α,γ, and r_α, shown in Figure 2. A summary of the mean values for the individual variables is presented in

Table 6. Summary of average values for the first group of indicators.

Variable	Substrate S [μm]	SD	Substrate U [μm]	SD	Substrate T [μm]	SD
Sα	15.73	4.75	26.22	13.75	20.91	5.17
Sγ	15.16	4.09	22.17	11.86	17.18	4.45
Rα	5.91	1.17	6.95	5.31	7.86	0.81
rα	7.74	3.85	14.42	7.74	8.58	1.73

, while the NIR test results are presented in

Table 7. Summary of NIR analysis results for the first group of indicators.

The Marked Differences Are Significant with p < 0.05000
	Sα	Sγ	Rα	rα
S-U	0.000745	0.007489	0.000215	0.000216
S-T	0.046223	0.360305	0.019301	0.563859
U-T	0.071475	0.360305	0.050120	0.000921

. The results show that for each of the variables considered, there is a statistically significant difference between the S and U-type substrates. In the comparison of the S and T cutters, the difference is statistically significant only for the S_α and R_α wear indicators. Comparing the mean values for the U and T cutters, the differences between the above-mentioned variants are even less pronounced, as they are only noticeable in the case of the rounding radius r_α determined from the clearance surface. In summary, it can be seen that a brand-new U-type blade shows initial signs of wear, indicating problems that may have occurred during tool preparation.

The second group of analyzed variables included indicators defining the asymmetry of the tip radius: D_α, A_α, nq, and apex n. The mean values for individual variables are presented in

Table 8. Summary of average values for the second group of indicators.

Variable	Substrate S [μm]	SD	Substrate U [μm]	SD	Substrate T [μm]	SD
Dα	3.30	0.68	5.10	2.33	4.37	0.91
Aα	2.98	1.79	13.92	14.64	5.42	2.46
nq	1.93	1.32	3.46	3.41	2.65	1.29
apex n	0.53	1.26	1.71	1.81	0.89	1.17

, while the NIR test results are presented in

Table 9. Summary of NIR analysis results for the second group of indicators.

The Marked Differences Are Significant with p < 0.05000
	Dα	Aα	nq	apex n
S-U	0.002802	0.000166	0.018000	0.024693
S-T	0.094327	0.299346	0.098959	0.413159
U-T	0.098601	0.002397	0.323519	0.112305

. As was the case with the analysis of wear indicators, statistically significant differences between S and U were observed for all variables considered. However, with the exception of the Aα indicator, no statistically significant difference was observed between substrates S and T. The data indicate that in the case of substrate U, there was a very significant shift of the fillet apex toward the flank, as evidenced by the change in the apex n value from approximately 0.5 μm (S) to approximately 1.7 μm (U). This was also accompanied by statistically significant changes in the remaining indicators presented in Table 8.

The third group of analyzed variables included indicators describing the operating conditions of the cutting edge. The mean values for each variable are presented in

Table 10. Summary of average values for the third group of indicators.

Variable	Substrate S [μm]	SD	Substrate U [μm]	SD	Substrate T [μm]	SD
PAα	2.21	2.08	11.12	10.68	4.11	2.21
Lα	4.82	1.48	9.89	6.17	6.40	1.77

, and the NIR test results are presented in

Table 11. Summary of NIR analysis results for the third group of indicators.

The Marked Differences Are Significant with p < 0.05000
	PAα	Lα
S-U	0.000054	0.000120
S-T	0.280552	0.138311
U-T	0.000958	0.005509

. Statistically significant differences were observed in the contact length of the cutting edge with the material on the flank side, L_α, and the volume of material crushed during machining between variants S and U. For example, L_α for the S blade was approximately 2 μm, while for the U blade it was as much as 11 μm. Such significant differences in this respect could have contributed to the deterioration of the operational properties of the coating applied to the U substrate.

In summary, both the differences in the initial tool wear indicators recorded for a brand-new tool and the less favorable operating conditions resulting from the change in the Lα parameter value could have caused the significantly poorer performance of the tool based on the U substrate compared to the others in the durability tests. Furthermore, the significant changes in the rounding asymmetry in the case of the mentioned variant (U) compared to variant S could have additionally led to an unfavorable stress distribution in the coating, thus causing its premature failure.

According to the authors, the key factors that caused the largest differences between the S and U substrates were disruptions in the manufacturing process. These can have a much greater impact on the operational properties of the tools than the microstructure of the tool material itself. In mass production conditions, such as the production of replaceable inserts for milling cutters, maintaining perfect repeatability of the cutting edge rounding is practically impossible. This is influenced, among other things, by the microstructure of the cemented carbide itself, where there is a very high risk of chipping and grain loss at the tip during the production process, for example, during finishing.

The difficulties associated with ensuring optimal cutting edge shape are evidenced by the high standard deviation values for individual indicators describing its microgeometry, clearly visible in the case of the U-type substrate. Consequently, a coating applied to this type of substrate will not fully fulfill its intended purpose. It should be noted that an attempt to explain the causes of this phenomenon based on microstructure and surface morphology studies was undertaken in this article. Unfortunately, it did not provide a fully satisfactory answer.

4. Conclusions

○

Coatings applied to substrate types S (ultra nano fine) and T (micro) significantly improved the durability of the tools by about 50%.

○

The wear of the coatings during tribological tests was an order of magnitude lower than that of the substrate alone. There is a clear advantage of the coating applied to the S substrate during wear tests compared to the other variants.

○

No significant differences were found in the nanohardness and adhesion of the coatings to the substrate—they were similar.

○

The reduction in the durability of the coating produced on the U substrate was influenced by the microgeometry of the substrate used because the conditions for producing the coatings were the same, and their thicknesses and other physico-mechanical properties were similar.

○

Research results show clearly that tool manufacturers should pay greater attention to optimally preparing the microgeometry of blades intended for coating, using micro-abrasive processing methods such as drag finishing and micro-blasting (wet or dry). As the literature shows, these methods enable significant improvements in tool properties.