TiB 2 embedded borosilicate coatings with improved wear resistance

Introduction

Devitrification of precursor glass ceramic (GC), i.e. frit deposited on metals is a well-established method for obtaining corrosion and wear-resistant surfaces [1]. The frit is applied to the metal by dipping, spraying, and electrostatic deposition techniques. Frits contain crystal nuclei embedded in a glass matrix and can be converted into a GC through controlled crystallization [2-4]. Because of their widespread industrial application in oven interiors and trays, architectural panels, and chemical storage tanks, research on GC coatings (GCCs) has grown [1,5,6]. In particular, the wear resistance of the GCC is of utmost importance, as breaking off the GCC during use removes the barrier of the anti-corrosion layer and thus the metal rust of the substrate [7-11]. GC materials are mechanically degraded by brittle fracture mechanisms due to crystal and residual glass phases [12]. Depending on the propagation of the crack, different fracture mechanisms occur in these materials: (i) trans-granular (crack propagation both in the glass and in the crystal phase), (ii) pull-out effect (fracture of the material during crack propagation in the nano-scale crystals) and (iii) intragranular (crack propagation only in the rest of the glass matrix). Uniaxial and inhomogeneous crack source imperfections in the matrix lead the material to shatter during stress, which triggers the brittle fracture phenomenon [13]. The wear qualities of GCs are influenced by the abrader material. Metal/GC contacts result in mild wear behaviour with reduced wear rate, cracks, and flakes because the metal sticks to the glass ceramics and sliding occur at the metal–metal interface. However, because delamination occurs from both sides and abraded particles also contribute to the wear in ceramic/GC contacts, the wear becomes sub-surface abrasion rather than surface abrasion, resulting in a higher number of fractures, flake forms, and pores [14].

However, there are few techniques for increasing the wear resistance of GCCs. Using additives to change the chemical composition of the GC mixture is one option. The overall porosity and porosity dimension distribution in the GC layer were modified by Rossi et al. [15] on the premise that the wear mechanism begins at the pores. The addition of hard ceramic particles (HCP) to the GC matrix is another approach on which the current research based. The prerequisites for HCP selection are listed as follows: (i) HCPs must withstand the crystallization temperature of the GC, (ii) must have a good affinity for the glass matrix, and (iii) should not be agglomerated [10]. Earlier studies on the integration of HCP into the glass matrix were carried out on GCs with a low crystallization temperature (∼500°C) and applied to the aluminium surface [7,9]. Rossi et al. [7] reported the influence of HCPs on the mechanical and chemical behaviour of GCs on aluminium surfaces. The authors showed that a high density of WC particles (15.63 g cm^–3) caused agglomeration in the matrix and reduced wear resistance. However, applying sonic stirring to the slurry dispersed the HCPs homogeneously. The production of wear-resistant GCCs that reduce costs and rework at the manufacturer's site mandates the establishment of advanced materials. The investigation of an alternative GC system reinforced with HCP is the subject of this article.

TiB₂ is a ceramic-based material with a relatively low density (4.52 g cm^–3) and a high degree of melting (3225°C). It is preferred in industrial applications due to its high hardness (>22 GPa), wear resistance, oxidation resistance up to 1000°C, and high electrical conductivity (9.0 × 10⁶ Ω.cm) [16,17]. The GCCs examined in this work are obtained by controlled crystallization of frits at 750–850°C. TiB₂ is added to the frit during the milling process. Therefore, TiB₂ is expected to be exposed to an intense oxidizing environment. Under the influence of the temperature in the oxidizing environment, the TiO₂ phase is formed in the outer and amorphous B₂O₃ in the inner layer of the TiB₂ surface [18] as in Equation (1) [17,18]. (1)

The B₂O₃ is in solid form at 450°C and after this temperature, it melts. The resulting B₂O₃ is amorphous, which prevents TiB₂ particles from interacting with the air environment, therefore preventing further oxidation. In this work, wear experiments were conducted on both metal/GC and ceramic/GC contacts because metal/GC interaction produces acceptable wear results, whereas ceramic/glass ceramic contacts induce severe wear behaviour in terms of mass loss driven by flakes [19]. The impact of the TiB₂ addition on GCC wear resistance was investigated in this study.

Materials and methods

In the present work, a well-established glass ceramic system of iron-containing sodium borosilicate was selected. TiB₂ HCP with a particle size of D₉₀=48 µm was purchased from Pavezyum Advanced Chemicals (Istanbul, Turkey). TiB₂ HCPs were systematically increased from 0.06 to 0.30 wt-%. DC06EK quality sheet steel was used as a substrate. To maintain the aesthetic appearance the addition of 2.5 wt-% black pigments (Fe–Co–Cr) to the chemical composition of frit was retained. The codes and contents of the prepared samples were provided in Table 1.

OPEN IN VIEWER

Table 1

The codes and contents of the samples.

Sample name	Amount of TiB2 addition (wt-%)
T0	None
T1	0.06
T2	0.12
T3	0.18
T4	0.24
T5	0.30

Steel substrate surfaces were activated by subsequent sandblasting, rinsing, and drying steps. The GCCs were applied by an electrostatic powder spray process to steel surfaces, resulting in a coating thickness of approximately 150 ± 50 μm. Subsequently, the samples were preheated to 550°C for 5 min and fired to 830°C in a box furnace (Protherm PLV 110/30) for 5 min.

The microstructure of the GCCs was observed by Jeol JSM-6060LV scanning electron microscopy (SEM) equipped with an energy dispersive X-ray (EDS) analysis. For SEM, an acceleration voltage of 20 kV was used at a working distance of 9.5 mm. In secondary electron yield imaging (SEI) mode image contrast is determined mainly by the sample morphology, whereas in backscattered electron imaging (BSE) mode the image contrast is set by the atomic number of the sample elements. To analyze the phase formations of the GCCs, Bruker XRD/D8 Advance X-ray diffractometer was used (Cu-K_α, 40 kV, 25 mA, 10°–70°, 0.002°, 1.5s).

The samples’ tribological properties were investigated by reciprocating wear tests in the CSM Tribometer M50 stainless steel balls (Ø10 mm) with a hardness of 62 Rc to determine the friction coefficient and wear ratio. The applied load was 5 N, and the wear tests were performed at a sliding speed of 30.16 cm sec^–1 for 4500 cycles with a 20 mm amplitude. Coupled sensors capture normal and lateral forces automatically, which are then sent to a computer system, which calculates the coefficient of friction over the entire distance. The 200 m test distance was chosen to determine the long-term trend of friction. Three tests were conducted for each tribo pair to obtain an average coefficient of friction. Wear rate is an important criterion since it can determine a material's wear behaviour based on the volume of material lost throughout the test and it can be calculated using the formula: (2) where K is wear rate in mm³ Nm^–1, V is wear volume in mm³, S is sliding distance in m and F is the amount of load in terms of N. Wear resistance was also evaluated via a two-body wear process by Taber test according to ASTM D4060-19 standard [11,20]. During wear testing, the Taber S-33 strip was used as an abrasive and the strips were affixed to the periphery of the CS-0 Wheel with a constant load of 500 g [21].

Results and discussion

To study the effect of TiB₂ additive on the GCC surface morphology SEM experiments were performed. SEM images of the reference sample were presented in Figure 1. No surface defects were visible on the GC-coated surfaces (see Figure 1(a)). The white web appearance was pigment particles based on the oxides of Fe-Co-Cr and was distributed on the surface. Pigments appear bright in both imaging modes since they are metallic oxides and are more conductive than the rest of the GC-matrix. In Figure 1(b) the dark grey area observed on the surface (dashed orange circle) is attributed to Al₂O₃ dense region, which is Al₂O₃ ball-mill sourced used in the milling process. Figure 1(b,c) represent the enlarged SEM image of the same sample. It was determined that chunk-shaped crystal was rich in Fe, Si, and O, which have been attributed to the iron silica crystals. Dark areas in both imaging modes were assigned to the glass matrix.

OPEN IN VIEWER

Figure 1

SEM images of the sample with addition of 0.12 wt-% TiB₂ addition (T2) (a) SEM image taken in SEI mode with ×1000 magnification. (b) Enlarged SEM image of the same sample, where EDS analysis was performed. (c) SEM image taken in BSE mode at the same spot.

In Figure 2 the effect of adding TiB2 HCP to the GC composite on the surface morphology is demonstrated. The EDS analysis showed that the crystals with a grain size of about 1 µm are pigment particles. Cr–Fe–Co-oxide-rich areas (white particles) on the surface are visible for all samples. The morphology of the samples represents slight changes with increasing TiB₂ addition. The homogeneous distribution of crystal particles due to the increase in surface tension with the increasing amount of amorphous B₂O₃ is prevented by the oxidation of TiB₂ during the controlled crystallization process [22,23]. The areas larger than 1 µm in size (coarser shape) are Fe, Si, and O elements rich. These coarse crystals are not pigments. Figure 3 shows the SEM cross-sectional images of the GCCs applied to the steel surface as a function of the addition of the TiB₂ HCP. The coating thicknesses were measured to be 150 ± 50 µm. Porosities are typical defects due to the development of gas during crystallization at elevated temperatures of around 770–850°C. The number and size of the porosities, however, depend on the crystal structure and the amorphous content of the frit [15,24-26]. The feature that determines the viscosity of glass and GC structures is the ratio of non-bridging oxygen atoms to tetrahedrally coordinated cations and the increase in modifier oxides in the structure, which increases the non-bridging oxygen rate and thus lowers the melt viscosity. Bubble formation is mainly caused by the viscosity of the GC. Low viscosity GCs result in the formation of larger bubbles, while high viscosity GCs result in the formation of smaller bubbles [25,26]. The increase in the additives in the GCC increases the viscosity of the coating; hence this behaviour leads to the formation of small bubbles. Larger bubble structures can be evacuated from the structure during crystallization, depending on the viscosity, or can form closed porosity by being trapped in the structure. Given the cross-sectional views, the addition of TiB₂ HCP does not significantly change the porosity of the GCC. In Figure 3, the insets confirm the adhesion between metal and GCC. The dendritic structures at the GC-metal interface point out that the electrolytic corrosion and chemical bonding mechanism of the coating are successfully achieved in all samples [27-29]. TiB₂ HCP addition did not change the adherence quality radically.

OPEN IN VIEWER

Figure 2

SEM images of the studied samples. From (a) to (f) increasing TiB₂ HCP mill addition is denoted.

OPEN IN VIEWER

Figure 3

Cross-section SEM images of the glass ceramic coated steel substrates (insets present the corresponding enlarged SEM images revealing the glass ceramic-metal interface).

XRD analyses on GC surfaces were performed to determine the crystal phase formations after the crystallization process and are shown in Figure 4. Three crystal phases have been detected. The main phases were (#PDF-04-013-7317) iron silicon oxide (Fe_2.72Si_0.28O₄) and (#PDF-00-047-0718) silicon dioxide (SiO₂) with an intermediate phase (#PDF-00-047-1777) pseduorutile (Fe₂Ti₃O₉). Iron silicon oxide is a phase observed in SiO2–Fe₂O₃–CaO and SiO₂–Al₂O₃–Fe₂O₃–(R⁺)₂O–(R²⁺)O GC systems for use in magnetic and biomedical applications areas [2]. Fe₂O₃ modifies the electronic density of the bond states of silicon and oxygen atoms in the silica network. This reduces the non-bridging oxygen content in the network [30,31]. However, since this phase is also present in the reference sample, it does not influence the mechanical and chemical resistance properties of the GCCs examined. This phase is assigned to the coarser Fe–Si-rich particles in Figure 1(b,c). The quartz crystal phase is common in most GC systems and has mechanical, thermal, and chemical resistance [32,33]. Quartz crystal was observed in all GC samples examined. Pseudorutile semi-crystalline phase is an intermediate phase between ilmenite (FeTiO₃) and TiO₂ (rutile) phases [34]. As can be seen from the coexistence of pseudorutile and iron silica phases, the main phase and the nucleating agent in the structure was iron in all GC structures [2].

OPEN IN VIEWER

Figure 4

XRD patterns of the studied glass ceramic samples.

Figure 5(a) shows the average friction coefficients (black columns) and wear rates (grey columns) of the studied samples. The coefficient of friction (COF) was 0.47 µ in T0 without TiB₂ addition, but it was 0.54 µ in the T5 after a 15% increase. The resistance of the TiB₂ HCPs in the sliding behaviour during the test is thought to be the cause of COF increase. The slight increase in COF in the case of TiB₂ embedded GCs can be explained by the cohesion of the added HCPs into the matrix. The cohesion of the HCPs was expected phenomena owing to the oxidation products of the TiB₂ particles, amorphous B₂O₃, promoted the cohesion of the HCPs into the GC matrix.

OPEN IN VIEWER

Figure 5

Average friction coefficient and wear rate (a) and SEM images of the ball-on-disc wear traces (b) of the studied samples.

With increasing TiB₂ HCP, the wear rates of the samples considerably decreased, which has supported that the TiB₂ added glass ceramic coated surface had better wear resistance than the reference coating without TiB₂ addition.

Figure 5(b) shows SEM images of the samples after ball-on-disc wear tests. Wear traces that appear after the test are known as post-analysis characteristic traces. The coating of T0 has been worn to a large extent. The large wear trace in T0 is considered to be caused by the repeated sliding of the reciprocating motion and relatively higher stress close to the surface. Larger wear traces are more prone to produce large flakes and wear particles from delamination resulting a higher wear rate [35]. The preliminary resistance to abrasion in the T1 is asserted by the reduction of the wear trace. The TiB₂ HCPs have shown to be advantageous even after the first addition, as the decrease in the wear trace is associated with the slight increase in the friction coefficient and the slight decrease in the wear rate. As the ratio of TiB₂ particles increased, shallowing of the grooves, and narrowing of the wear traces observed. Under constant sliding parameters, narrowing of the wear trace implies that the stress on the surface is reduced thus has created lower flake and wear particle delamination, which yielded a lower wear rate. Thus, the flake amount decreased from T0 to T5.

Figure 6 shows the mass losses of the samples as a function of the TiB₂ HCP addition following the Taber abrasion test. With increased TiB₂ HCP additive, mass loss reduces (in Figure 6a), which indicates a well-wear-resistant surface. Figure 6(b) demonstrates the post-abrasion SEM images of the studied samples. The regions indicated by the arrows are the pores that are opened as a result of the abrasion test, and negatively affect the wear resistance properties of the samples. The wear resistance is directly dependent on the stress points in GCCs. The higher the stress point, the lower the wear resistance. The parameters that create the stress points are porosity, micro and macro cracks, and agglomerated particles within the glass matrix. Rossi et al. [15] showed that mill additives adjust the viscosity which in turn the pore dimension within the glass matrix. Pores contain high levels of local stress. Owing to the abrasive effect of sandpaper discs, flakes loosen from the structure and cracks spread. The traces created by the abrasive discs are shown as abrasion traces. The lines shown in T2 indicate crack propagation. Vitreous flakes that resulted from the rotational movement of the abrasion behaviour are the characteristic result of the brittle fracture mechanism. T5 with the highest TiB₂ content (0.30 wt-%) among other samples shows the highest wear resistance. As the amount of the glass matrix increased gradually, which is most probably caused by the amorphous B₂O₃ (formed by oxidation of TiB₂ HCPs), the residual stress has decreased. One of the reasons for the gradually decreasing mass loss is the decreasing areas of stress due to the relaxation of residual stress and promotion of densification [36-39]. Besides, the presence of TiB₂ HCPs in the structure contributes to the development of wear resistance of the entire surface via the regional resistances in the surface by the properties of hardness and toughness. T5 shows the highest resistance in all tests and the wear resistance increase with the increase of the TiB₂ ratio. In this study, TiB₂ particles oxidized in the matrix are terminated by the amorphous oxidation product B₂O₃ before they are wetted by the GC matrix. While the oxidation of TiB₂ is prevented, the amorphous B₂O₃ formed is fused with the glass matrix in the GC structure.

OPEN IN VIEWER

Figure 6

Mass loss (a) and SEM images (b) of the studied samples after the Taber abrasion test.

Conclusion

In this work, the effect of TiB₂ HCP on GCCs surface properties was investigated. Wear resistance was explained in terms of change in colour, gloss, morphology, and mass loss. The incorporation of TiB₂ HCP into the glass matrix did not change the overall crystallinity of the samples. The amorphous B₂O₃ compound has eliminated the oxidative environment by wetting the TiB₂ HCP particles. The B₂O₃ compound was combined with the glass matrix and has reduced the stress areas in the structure. Also, during the incorporation of the HCPs into the amorphous matrix, the tension at the matrix-particle interface has been eliminated. With decreasing stress zones, the wear resistance of the coatings was developed. The reduction in mass loss from 2.3 to 0.2 mg, in wear rate from 24.573 to 4.205 mm³ Nm^–1 and augmentation in coefficient of friction from 0.47 to 0.54 µ with increasing TiB₂ HCP was translated into an increased wear resistance.