Surface Treatments on Cobalt–Chromium Alloys for Layering Ceramic Paint Coatings in Dental Prosthetics

Abstract

Ceramic dental prosthetics with internal metal structures are made from a cobalt–chromium alloy that is coated with ceramic. This study aims to validate surface treatments for the metal that enhance the adhesion of the ceramic coating under masticatory forces. Surface conditioning is performed using mechanical methods, like sandblasting (SB), and thermal methods, such as oxidation (O). The ceramic coating is applied to the metal component following the conditioning process, which can be conducted using either a single method or a combination of methods. Each conditioned sample undergoes characterization through various techniques, including drop shape analysis (DSA), scanning electron microscopy (SEM), X-ray diffraction (EDX), and atomic force microscopy (AFM). After the ceramic coating is applied and subjected to thermal sintering, the metal–ceramic samples are mechanically tested to assess the adhesion of the ceramic layer. The research findings, illustrated by scanning electron microscopy (SEM) images of the metal structures’ surfaces, indicate that alloy powder particles ranging from 10 to 50 µm were either adhered to the surfaces or present as discrete dots. Particles that exceed the initial design specifications of the structure can be smoothed out using sandblasting or mechanical finishing techniques. The energy-dispersive spectroscopy (EDS) results show that, after sandblasting, fragments of aluminum oxide remain trapped on the surface of the metal structures. These remnants are considered impurities, which can negatively impact the adhesion of the ceramic to the metal substrate. The analysis focuses on the exfoliation of the ceramic material from the deformed metal surfaces. The results emphasize the significant role of the sandblasting method and the micro-topography it creates, as well as the importance of the oxidation temperature in the treatment process. Drawing on 25 years of experience in dental prosthetics and the findings from this study, this publication aims to serve as a guide for applying the ceramic bonding layer to metal surfaces and for conditioning methods. These practices are essential for enhancing the adhesion of ceramic materials to metal substrates.

1. Introduction

The novelty of this study lies in its holistic approach to the subject in question. The application of ceramic coatings on dental alloys presents challenges when performed in a dental laboratory. The adherence of dental ceramics to metal frameworks is influenced by the individual experiences of each dental technician. Why is this the case?

Manufacturers of alloy materials specify in their guidelines that dental technicians must follow precise instructions regarding the ceramic materials to be used. Similarly, ceramic material producers provide guidelines that outline the necessary procedures for the surface conditioning of the alloys, which must be tailored to the specific types of metal being used.

In general, to ensure good adhesion, the metal framework should undergo heat treatment to relieve tension and should be sandblasted using aluminum oxide. The surface must then be coated with a bonding ceramic layer followed by the application of the ceramic material, which will progressively increase the thickness of the coating.

We combined 25 years of practical experience in dental laboratory work with research in material science to establish a protocol aimed at achieving the optimal adhesion of dental ceramics to cobalt–chromium dental alloy frameworks, which can be used in dental laboratory practice as a guideline.

Cobalt–chromium-based alloys have a long history of use in dental prosthetics [1]. The traditional manufacturing process involves the handcrafted waxing, melting, and molding of the alloys, followed by the manual application and sintering of ceramic dental materials [2,3,4,5,6]. This process requires skilled dental laboratory technicians with specialized practical abilities. Recently, traditional crafting skills have evolved into digital techniques, and there has been significant technology transfer from industrial practices to small-scale dental prosthetic production [7,8,9]. Advanced Additive Manufacturing (AM) technology is radically changing operations in dental laboratories; however, cobalt–chromium alloys remain a preferred material in dental prosthetics [2,7]. Powder technology and the selective laser melting (SLM) method are among the most commonly used techniques for manufacturing metal dental frames [9,10]. The durability and functionality of metal–ceramic dental prostheses depend on the strength of the bond between the two materials, which can vary based on the surface treatment applied to the frame before applying the ceramic coating [11,12]. Manufacturers of dental materials must align the properties of dental ceramics with those of dental alloys to ensure compatibility and synergy and to enhance the adhesion of ceramic layers to the metallic framework [13,14,15,16].

The instructions for using SLM dental alloys specify that thermal treatment is necessary after construction to relieve any residual stress in the material. Once the frame is detached from the platform, it must undergo sandblasting [17]. If a heat treatment is performed, any oxides formed on the metallic surface need to be removed. The frame’s surface should be cleaned using a steam blaster or by boiling it in distilled water. After this cleaning step, it is important not to touch the surfaces with one’s bare hands; instead, arterial clamps or similar tools should be used for handling. Additionally, it should be ensured that the frames are properly supported during the firing process [18].

According to ceramic material manufacturers, it is recommended to apply ceramic bonding material immediately after surface treatments [19], such as finishing, sandblasting, and cleaning the metal framework [20]. This micro-fine layer enhances the adhesion of the ceramic material to the metal framework, protects against debonding, and improves overall reliability [21]. Bonding materials [22,23] help to address differences in the expansion coefficients between the metal and ceramic, and they prevent the escape of metal oxides. This is particularly important in mitigating issues that can arise when using alloys that tend to form a strong oxide layer [24,25].

Traditional dental ceramics are primarily feldspar-based and consist of a significant amount of feldspar (KAlSi₃O₈), along with quartz (SiO₂) and kaolin (Al₂O₃·2SiO₂·2H₂O). Feldspar is a grayish crystalline mineral commonly found in iron- and mica-rich rocks. To create the ceramic material, feldspar rocks are ground and passed through strong magnets to remove iron compounds, resulting in a pure powder. Quartz, or silica (SiO₂), makes up 55%–65% of the mixture and is responsible for the translucency of the dental restoration. Since quartz is not a particularly strong material, 20%–25% alumina (Al₂O₃) is added as a reinforcing agent. Kaolin, a hydrated aluminum silicate, is included in a small amount (about 4%), as it has opaque properties unlike the translucency of natural human teeth. Kaolin’s primary role in the composition of dental ceramics is to bind the loosely held ceramic particles together [26].

In addition to defining the mechanism of metal–ceramic bond strength, evaluating the adherence of dental ceramics is important for studying the fracture mode of the SLM metal–ceramic system when plastic deformation forces are applied [27].

The wetting of the surface is an important criterion in many industrial processes, such as coating and printing. In these applications, both the liquid’s surface tension and the solid’s properties must be optimized to ensure proper adhesion between the liquid and solid surfaces. The solid material should be designed to selectively absorb the liquid, a characteristic influenced by its wetting properties. Wetting is typically assessed using the contact angle, which is defined as the angle formed between the tangent to the liquid–vapor interface and the solid surface at the point where the three phases meet. By convention, the contact angle is measured from the liquid side. In this protocol, the method used to measure contact angles is called sessile drop goniometry. This technique involves recording a video of a water droplet on a solid surface and then determining the contact angle from the images in the video through a fitting procedure [28].

2. Materials and Methods

2.1. Materials

The first step in Additive Manufacturing is the Computer-Aided Design (CAD) stage. Meshmixer software Version 3.5 (Autodesk. Inc., San Francisco, CA, USA) was used to make a digital design for 30 × 10 × 0.7 mm plate samples, which was then saved as a stereo lithographic (STL) file (Figure 1a). The STL file was opened using 3Shape CAMbridge CAM Software Version 8.1  [29], and after creating 10 true duplicates of the main plate-shaped samples, all 10 plates were fitted on the space of the virtual working platform. The structures displayed on the virtual working platform are shown in Figure 1b, along with the research samples, including the dental prosthetic frameworks created through daily printing. This indicates that the same material is used for both types of pieces and intended for use in the oral cavity  (Figure 1b).

The 3D designs of all 10 plate-shaped specimens from the virtual working platform were produced using an SLM manufacturing machine (Mysint100 DualLaser, Sisma S.p.A, Vicenza, Italy), 2× Fiber Laser 200 W source, Quartz F-Theta Lens precision optics, 2 × 55 μm laser spot diameter, and typical layer thickness 20 μm–40 μm (adjustable/regolabile) in Nitrogen, Argon–Azoto, Argon inert gas atmosphere.

Cobalt–chromium-based alloy powder (Mediloy-SCo, BEGO Gmbh & Co. KG, Bremen, Germany) was the used material and Sisma S.p.A-recommended material for the SLM MySint100 Dual Laser machine. The specifications were in accordance with the manufacturer’s recommended standards, and the composition of the metal powder is shown in

Table 1. The composition in % by weight of metal powder (Mediloy SCo).

Chemical Elements	Co	Cr	W	Mo	Si
wt%	63.9	24.7	5.4	5.0	1.0

BEGO Mediloy-SCo powder alloy is recommended by Sisma S.p.A. to be used for Mysint 100 DualLaser machine; coefficient of thermal expansion (CTE) of 14.0 (25–500 °C, 10⁻⁶ K⁻¹).

.

In accordance with the manufacturer’s instructions for use, stress relief heat treatment was performed using a heat treatment furnace (Miditherm 200, BEGO Gmbh, Bremen, Germany). The removable part of the production platform was inserted in the furnace at a temperature of 650 °C. The temperature was increased to 800 °C within 12 min and was held for 15 min. Next, the temperature was decreased to 550 °C within 15 min. The platform was removed from the pre-heating furnace at 550 °C (or below) for further processing.

After the stress relief heat treatment and cooling, the plate-shaped samples were removed from the platform.

All 10 plate-shaped samples were clasped in surgical forceps and prepared for cleaning one by one with hot steam using a steam blaster machine (EGV 28 AI, EUROCEM, Milan, Italy) and subsequently cleaned in a ultrasound bath for 5 min/room temperature, 21 °Celsius (Easyclean-Renfert Dental, Hilzingen, Germany).

All the samples were left to rest on absorbent paper towel.

All 10 samples received surface treatments: sandblasting (SB) with aluminum oxide particles (250 µm/3–4 bar), 60 s/sample using a metal sandblasting machine, until the surface had a matte texture (Easyblast-BEGO, Bremen, Germany); high-temperature treatment (T = 900 °C, vacuum) (HTT) until an oxide layer formed, using a ceramic furnace (Programat P300-IVOCLAR, Vivadent, Lichtenstein); mechanical processing (PM) by polishing with abrasive materials using a brush with abrasive Cr₂O₃ paste (Green Paste-Carbochim SA, Cluj-Napoca, Romania) and fluff, using a dental micro motor (Forte 100A, Shaeshin, Republic of Korea); and one layer of ceramic bonding (CB) and three layers of opaque paste (PO) brushed on in successive layers (Initial MC, GC Corporation, Leuven, Belgium), as shown in

Table 2. Surface treatments on cobalt–chromium-based alloy plate-shaped samples (P).

P	SB250-1	MP	HTT	SB250-2	CB	PO-1	PO-2	PO-3
P1	x	-	-	-	-	x	x	x
P2	x	-	x	-	-	x	x	x
P3	x	-	x	x	-	x	x	x
P4	x	-	x	x	x	x	x	x
P5	x	-	-	-	x	x	x	x
P6	x	-	x	-	x	x	x	x
P7	-	x	-	-	-	x	x	x
P8	-	x	x	-	-	x	x	x
P9	-	x	x	-	x	x	x	x
P10	-	x	-	-	x	x	x	x

(x)—surface treatment application; (-)—treatment not applied to surface.

.

2.2. Methods

2.2.1. Scanning Electron Microscopy and Energy Dispersive Spectroscopy Analysis

All 10 samples were analyzed using scanning electron microscopy (S-4800, Hitachi, Japan), which produces images of a sample by scanning the surface with a focused beam of electrons. Energy-dispersive spectroscopy (EDS) analysis, used together with SEM and an X-ray detector, generates information about the chemical composition of a sample, including what elements are present as well as their distribution and concentration. All dates were processed with AzTEC Software LLC, Morristown, NJ, USA.

2.2.2. Optical 3D Scanning for Surface Roughness Characterization

To characterize surface roughness, the samples were analyzed using a 3D scanning measurement system. Alicona Infinite Focus G5 (Miltera-Ontario, Cambridge, ON, Canada) equipment is based on only one optical sensor and provides surface roughness measurement and shape measurement, particularly for smooth and highly polished surfaces. Operating within the µm and sub-µm range, this device provides precise results regardless of material geometry or surface finish.

2.2.3. Drop Shape Analysis and Surface Contact Angle

The analysis and measurement of the contact angle between ultrapure water droplets and samples with different surface treatments were conducted using a Drop Shape Analysis System [30]. The data were processed with the dedicated software ADVANCE (https://www.kruss-scientific.com/en/products-services/advance-software/advance-drop-shape, accessed on 8 July 2025, Kruss GmbH, Hamburg, Germany). For the analysis of 2 µL droplets, 300 contact angle measurements were taken using the elliptical method at a frequency of one measurement per second, maintained at a temperature of 23 °C.

2.2.4. Mechanical Tests

The delimitation process of the ceramic layer in relation to the base material can be experimentally demonstrated using bending stress. In this setup, the base material is represented by a bar with a rectangular cross-section that maintains a constant stiffness (E·Iz = constant). One end of the bar is fixed, while the other end is free, as illustrated in Figure 2. The bar is subjected to plane bending by applying a concentrated force (F) at its free end. In this context, E·Iz denotes the stiffness modulus of the specimen under stress, where E refers to the longitudinal elasticity modulus (or Young’s modulus), and Iz represents the axial moment of inertia of the bar’s cross-section relative to the reference axis (z).

Figure 3 illustrates the experimental setup, which is outlined as follows: The sample (1) is securely fixed to the metal base plate (5) using magnetic supports (6). The displacement of the free end of the sample is measured with a digital display comparator (3).

To account for any vertical movement of the specimen’s fixing element, an analog display comparator (4) is employed.

The bending force F (as shown in Figure 1) is applied using a plate (7) onto which a series of calibrated weights (8) is sequentially placed.

The force F is calculated using the following mathematical relationship:

$$F=m·g$$

(1)

where m is the mass expressed in [kg], and g is the gravitational acceleration, expressed in [m/s²].

The ceramic layer is placed on the upper fiber of the base material to allow for easy observation during bending tests. In this arrangement, the ceramic layer is subjected to tensile stress. The deformation behavior of the sample can be described using the equation of the elastic curve [31,32,33,34].

$$v={\displaystyle \frac{F·{L1}^3}{6EIz}}·\left({\displaystyle \frac{{3x}^2}{{L1}^2}}-{\displaystyle \frac{x^3}{{L1}^3}}\right)$$

(2)

where x represents the distance from the fixed end of the specimen to the section where the vertical displacement is measured.

The specific deformation ε of the ceramic layer can be expressed by the following relation:

$$\mathsf{\epsilon }={\displaystyle \frac{y}{⍴}}$$

(3)

where y represents the distance from the centroid of the cross-section of the base material to the ceramic layer, and ρ is the radius of curvature of the sample.

$${\displaystyle \frac{1}{⍴}}=\pm {\displaystyle \frac{\frac{d^{2}v}{d{x}^2}}{{\left[1+{\left(\frac{dv}{dx}\right)}^2\right]}^{\frac{3}{2}}}}$$

(4)

In general, within the domain of elastic deformations, the rotation of transverse sections is considered negligible. Consequently, the mathematical relation in (4) can be expressed in the following form:

$${\displaystyle \frac{1}{⍴}}={\displaystyle \frac{d^{2}v}{d{x}^2}}$$

(5)

The mathematical relation in (5) can also be written as follows:

$${\displaystyle \frac{1}{⍴}}={\displaystyle \frac{Mi}{EIz}}$$

(6)

where M_i is the bending moment, which can be expressed as the product of the force F and the distance from the point where the force F is applied to the analyzed section.

From the mathematical relations in (3) and (6), the following can be obtained:

$$\epsilon ={\displaystyle \frac{Mi}{EIz}}·{\displaystyle \frac{h}{2}}$$

(7)

3. Results

3.1. Scanning Electron Microscopy and Energy-Dispersive Spectroscopy Results

The SLM fabrication process was tracked from the raw material to the final product. As shown in SEM images (Figure 4a), the powder particles and their granulation size range are found to be between 11 µm and 34 µm and 47 µm, and the EDS analysis (Figure 4b) of the cobalt–chrome-based alloy powder confirms that the manufacturer’s usage instructions described the weight percent of cobalt, chromium, molybdenum, and Wolfram in the mass alloy Co61Cr25.3Mo6W5.4.

EDS elemental distribution maps are shown in Figure 5.

After the process of selective laser melting (SLM), where powder particles were fused using a dual-laser system to create a solid metallic sample (denoted as P), scanning electron microscopy (SEM) images were captured. A surface analysis of the ten samples of P revealed that alloy powder particles, measuring between 10 and 50 µm, were either adhered to the surfaces or present as discrete dots (see Figure 6a–d). To enhance the surfaces and remove the powder particles, the metallic P samples underwent sandblasting with aluminum oxide (Al₂O₃) at a diameter of 250 µm and a pressure of 3–4 bars. The energy-dispersive spectroscopy (EDS) analyses conducted before and after the sandblasting process indicated that some Al₂O₃ remained on the surfaces of the P samples (refer to Figure 6e,f).

The EDS quantitative spectrum indicates that the sandblasted surface of the P samples contains 9.1 wt% aluminum. However, when the surfaces of the P samples were mechanically polished (MP) to achieve a mirror-like finish, aluminum was removed from the surfaces of samples P7, P8, and P10.

The sandblasted surfaces of the P2, P3, P4, and P6 samples were treated with high-temperature treatments (HTTs), resulting in the presence of aluminum on the surfaces at a level of 6.5 wt%. Similarly, the mechanically polished surfaces of the P8 and P9 samples also underwent high-temperature treatments (HTTs). In both cases, whether sandblasted or polished, the increase in oxygen content ranged from 24% to 25.5 wt%. The results are displayed in Table 2.

The high-temperature-treated surfaces of the P3 and P4 samples were sandblasted a second time with aluminum oxide (250 µm, 3–4 bar). The quantitative EDS spectra indicate that the amount of aluminum increased to 10.3 wt%, while the quantity of oxygen decreased to 8.7 wt%.

The EDS analysis revealed a high quantity of aluminum on the surface of P samples after the sandblasting process, and an SEM image of a broken Al₂O₃ particle (10 µm) is shown in Figure 7a. In the SEM image taken from the P sandblasted samples, which includes EDS dot mapping, we can observe the area corresponding to the sample component and aluminum (red) (Figure 7b). Additionally, there are individual maps for each component: oxygen (Figure 7c), aluminum (Figure 7d), cobalt (Figure 7e), chromium (Figure 7f), molybdenum (Figure 7g), and tungsten (Figure 7h).

The mechanical polished samples P8 and P9, which were treated at a high temperature of 980 °C, developed oxide layers on their surfaces (see Figure 8a,b). Similarly, the sandblasted samples P2, P3, and P4, also treated at 980 °C, exhibited oxide layers on their surfaces (see Figure 8c,d).

EDS spectra from the surfaces treated at high temperatures (980 °C) indicated the presence of 23.3 wt% O on mechanically polished surfaces with no aluminum detected and 23.9 wt% O on sandblasted surfaces with 4.7 wt% aluminum present (

Table 3. Chemical Composition in wt%.

Chemical Elements wt%	Co	Cr	Mo	W	O	Al
powder alloy (Mediloy S-Co, BEGO) based on instruction for use	63.9	24.7	5.4	5.0	-	-
powder alloy (Mediloy S-Co, BEGO) EDS results	63.0	25.3	6.0	5.4	-	-
SB sample	5.3	21.5	3.9	4.2	8.0	9.1
MP sample	64.1	25.2	5.3	5.1	-	-
SB + HTT sample	40.7	22.8	2.6	3.1	24.3	6.5
MP + HTT sample	45.1	22.2	3.3	3.8	25.5	-
SB + HTT + SB sample	52.4	20.7	1.9	4.0	8.7	10.3

).

The SEM images taken after the process of mechanical plastic deformation and delaminating the ceramic material from the metallic sample highlighted the adhesion differences in correlation with surface treatments (Figure 9).

3.2. Roughness Surfaces Profile

Surface roughness is an important result, even if the small irregularities on the surface of the material are caused during the manufacturing process or are integrated into the surface conditioning process.

After conducting 3D analyses of surface roughness, we developed profiles. The parameters R (Ra, Rq, Rz) and S (Sa, Sq, Sp, Sv, Sz) were obtained and are shown in

Table 4. Roughness parameters obtained using 3D scanning measurements.

	Unit	SB	SB + HTT	MP	MP + HTT
Number of data values	Elem.	826,336	1,004,757	2,242,323	1,526,855
Number of histograms	Class.	115	112	146	64
Mean data values	[μm]	0.077	0.048	0.020	0.065
σ	[μm]	0.863	0.861	0.598	0.635
Ra	[μm]	0.58	0.58	0.14	0.35
Rq	[μm]	0.73	0.73	0.18	0.18
Rz	[μm]	4.12	4.25	0.91	0.91
Sa	[μm]	0.685	0.677	0.445	0.480
Sq	[μm]	0.867	0.863	0.598	0.638
Sp	[μm]	5.141	8.856	5.858	9.192
Sv	[μm]	5.515	5.298	3.000	5.806
Sz	[μm]	10.656	14.155	8.858	14.998
S10z	[μm]	9.272	11.693	6.773	12.432
Ssk		0.134	0.127	−0.225	−0.561
Sku		3.337	3.797	4.653	6.425
Sdq		0.119	0.112	0.031	0.058
Sdr	%	0.674	0.608	0.046	0.164
FLTt	[μm]	10.656	14.155	8.858	14.998
Lc	[μm]	800.00	800.000	800.000	800.000

σ—mean value of standard deviation; Ra—arithmetic average height; Rz—ten-point height; Sa—average height of selected area; Sq—root mean square height of selected area; Sp—maximum peak height of selected area; Sv—valley depth of selected area; Sz—maximum height of selected area; S10z—ten-point height of selected area; Ssk—skewness of selected area; Sku—kurtosis of selected area; Sdq—root mean square gradient; Sdr—developed interfacial area ratio; FLTt—flatness using least squares reference plane; Lc—LambdaC: cutoff wavelength.

.

3.3. Drop Shape Analysis Results

The results of the contact angle measurements revealed significant differences in values depending on the surface treatment applied to the samples. During the drop shape analysis of the alloy samples, we noted mean contact angles (CA(M)) at both the initial contact of the droplet with the surface and after 300 measurements. ADVANCE Software calculates the average of all contact angles measured over a 300-s period. Additionally, we calculated the difference between the mean contact angle at the first and last measurements to evaluate the decrease in CA(M) over time (see

Table 5. Values for CA(M) measurements with drop shape analysis method.

Types of Surfaces	First Step CA(M)	Steps	Last Step CA(M)	Average CA(M)	ΔCA(M)
SB250 P1 and P5 (Figure 10a)	80.52	300	58.80	61.67 (±10.58)	21.72
SB250/HTT P2 and P6 (Figure 10b)	27.46	300	9.00	18.81 (±5.33)	18.46
SB250/HTT/SB250 P3 and P4 (Figure 10c)	88.48	300	61.23	64.80 (±16.00)	27.25
MP P7 and P10 (Figure 10d)	99.50	300	80.45	85.60 (±7.50)	19.05
MP/HTT P8 and P9 (Figure 10e)	58.59	300	34.16	37.82 (±12.44)	24.43

PPSSB250—sandblasted surface using aluminum oxide 250 µm; SB250/HTT—sandblasted surface and high-temperature treatment; SB250/HTT/SB250—sandblasted surface before and after high-temperature treatment; MP—mechanically polished surface; MP/HTT—mechanically polished surface and high-temperature treatment.

, Figure 10 and Figure 11).

Figure 11. CA(M) comparison diagram between different sample surfaces.

Figure 11. CA(M) comparison diagram between different sample surfaces.

3.4. Mechanical Test Results

The samples were subjected to a maximum force of 27,534 N during the final stage of loading. For sample P1, the maximum displacement in the vertical plane was 4.55 mm, while for sample P2, it was 4.075 mm. It was observed that in both cases, no delamination of the ceramic layer occurred, as illustrated in Figure 12a,b.

At this stage of loading, the rotations of the transverse sections can no longer be ignored, and it becomes difficult to maintain the position of the device applying the force \( F \). This leads to a potential shift in the direction of force application. For this reason, a qualitative evaluation, in addition to the previously presented quantitative assessment, is deemed important. When a load is applied that induces plastic deformations, the delaminating process is observed in all 10 samples.

4. Discussion

The analysis of SEM images demonstrates noticeable changes in the surface of the samples following mechanical treatments, including sandblasting with aluminum oxide and mechanical polishing to achieve a mirror-like texture. Additionally, significant alterations are observed on the surfaces of samples that underwent heat treatment at high temperatures, as Yan X. et al. show in their publication [35].

The manufacturer recommends sandblasting with aluminum oxide as a surface treatment for samples produced using the SLM method. This process effectively removes alloy powder particles that may adhere to the sample’s surface outside the intended geometry specified in the CAD or STL file, as other authors described in their publications [36,37,38].

Nishigawa et al. talk about the influence of the sandblasting process, and the SEM images of the sandblasted samples show that unwanted alloy particles have been removed, resulting in a uniform surface texture. EDS spectrum analyses of the sandblasted surface reveal a significant amount of aluminum remaining after the sandblasting process. Additionally, the SEM images taken from the area highlighted by the EDS spectrum map confirm the presence of broken pieces of aluminum oxide (10 µm) that are trapped within the shallow depths of the sample due to mechanical embedding [39].

The presence of broken and embedded aluminum oxide particles on the surface of the sample likely results from the sandblasting process. During this process, aluminum oxide particles (250 µm) are propelled at a pressure of 3–4 atm from the sandblasting machine, causing them to collide with the sample’s surface and fragment.

It is important to note that cleaning with pressurized steam and an ultrasonic bath did not effectively remove the aluminum oxide particles adhering to the surface of the samples. However, mechanical polishing successfully removed these aluminum oxide particles from the surface of the sandblasted sample, indicating their depth of inclusion within the surface.

Takaichi et al. showed in their publication that the heat treatment applied to the samples made using the SLM (selective laser melting) method is essential for relieving material tension [40]. After this thermal treatment, an oxide layer forms on the surface of the samples. If this layer is removed through sandblasting with aluminum oxide, it will increase the residual aluminum content on the surface of the samples.

The adhesion of ceramic coatings on metallic surfaces has been the subject of various experiments. In some cases, specific samples retained a layer of oxides on their surfaces even after undergoing sandblasting, resulting in different surfaces receiving ceramic coatings [41]. The adhesion of the coating is influenced by both the cleanliness and roughness of the underlying surface.

The roughness of sandblasted samples increases the total contact area between the coating material and the metal substrate, which enhances mechanical interlocking. However, this process may also result in a higher amount of residual aluminum being left on the surface. In contrast, mechanically polished samples have a mirror-like finish that results in a non-adherent surface. This polishing method effectively minimizes the presence of residual elements due to its superior cleaning capabilities.

Surface finishing refers to the process of modifying a metal’s surface, which can involve removing, adding, or reshaping material. Surface finish is a measure of the overall texture of a product’s surface, defined by three key characteristics: surface roughness, waviness, and lay.

The change in roughness parameters before the conditioning surface process indicates that the surface is changed by external actions. The arithmetic average height parameter (Ra) is different in value on the sandblasted (SB) surface (0.58 µm Ra) from the one on the mechanically polished (MP) surface (0.14 µm Ra). An interesting issue is that when a high-temperature treatment is applied to these two types of surface textures, only the Ra parameter for the MP surface indicates the changes on the surface and the modification of the roughness profile (0.35 µm Ra).

The S roughness parameters Sp, Sv, and Sz refer to the maximum peak height, maximum valley depth, and maximum height of the selected area, respectively. These parameters serve as indicators for changes in the surface properties of materials subjected to high-temperature treatment (HTT).

The S parameters for the sandblasted surfaces are quite well equilibrated (5.141 µm Sp, 5.515 µm Sv, and 10.656 µm for Sz). When the high-temperature treatment is applied to the sandblasted surface material, the Sz value grows to 14.155 µm. The value of Sp grew to 3.715 µm and the Sv value to just 0.217 µm. The difference between the sandblasted surface Sp value and the high-temperature treatment sandblasted surface Sp value is 3.499 µm.

The S parameter values for the mechanically polished surfaces are not equilibrated, showing values of 5.858 µm for Sp, 3.000 µm for Sv, and 8.858 µm for Sz. When high-temperature treatment is applied to the mechanically polished surface, the Sz value increases to 14.998 µm. Additionally, the Sp value increases to 3.334 µm and the Sv value to 2.806. The difference between the Sp values of the mechanically polished surfaces and the high-temperature-treated mechanically polished surface is 6.14 µm, double the value of that of sandblasted surfaces.

The wettability of a surface [42] is crucial for ensuring that ceramic coatings adhere firmly and securely to metallic samples. Analyzing the wetting effect through drop shape analysis and measuring the contact angle between a drop of ultrapure water and the surface of the samples can provide insights into new approaches to pre-coating surface treatments. Analyzing the contact angle reveals that sandblasted rough surfaces exhibit a higher degree of wettability compared to the smoother, mechanically polished surfaces. Additionally, applying a heat treatment process to mechanically polished samples improves their wettability, resulting in a smaller contact angle than that measured on the sandblasted surfaces.

The discussion can focus on the potential for enhancing the adhesion of the ceramic coating to the metal surface, even when the surface is mechanically polished rather than traditionally sandblasted. A heat treatment is applied to the sample, which generates layers of oxides. This process increases surface roughness and enhances the wetting phenomenon for polished mirror-finish samples that are free of aluminum inclusions.

After the plastic deformation of the samples, slices of the ceramic layer begin to delaminate, running parallel to the width of the samples. The delamination process is inversely proportional to the adhesion of the ceramic material to the metal substrate, and it is influenced by the surface treatments applied to each sample.

The samples that were mechanically polished to a mirror finish, even after undergoing high-temperature thermal treatment and receiving a layer of ceramic bonding, exhibited a significant level of delamination in the ceramic layers. The ceramic layers detached along with the oxide layer from the surface of the metal sample. This observation indicates the presence of a non-adherent oxide layer formed on the sample’s surface.

The samples that underwent sandblasting with aluminum oxide can be categorized into three groups: sandblasted samples, thermally treated samples, and samples with a ceramic bonding coating. The sandblasted samples alone showed delamination on the surface, exposing the underlying metal with a texture characteristic of the sandblasting process. In contrast, both the sandblasted and heat-treated samples exhibited similar delamination on the surface. However, the areas where each slice was joined to the metal substrate were clearly defined, indicating a stronger adhesion of the ceramic material to the metal samples with oxide layers on their surfaces.

The samples that were sandblasted, thermally treated, and coated with a layer of ceramic bonding exhibited the least delamination at the edges. Additionally, the contact angle measured on the sandblasted and thermally treated surface was the smallest among those of all the surfaces analyzed, indicating high wettability and a favorable environment for applying ceramic bonding.

5. Conclusions

The treatments applied to the surfaces of metal samples made from cobalt–chromium powder alloys using the SLM method significantly enhance the adhesion of ceramic layers to the base material.

The scanning electron microscopy (SEM) images combined with EDS diagrams and the results from DropShapeAnalyser showed the quality of the metallic surface conditioning process.

The increase in S parameter values after high-temperature treatment indicates oxidation on the cobalt–chromium surface and the growth of oxide layers measuring 3–6 microns.

The decrease in the contact angle between solid and pure water is linear because the rate of decrease in the contact angle remains constant throughout the measurement period. Notably, the contact angle on surface P7, which was mechanically polished to a mirror finish, stabilized after 300 measurements, indicating a plateau. In contrast, surfaces P1 and P3, which underwent sandblasting as their final treatment, reached the same contact angle after 300 measurements. Meanwhile, the contact angle measurement for sample P8—treated with mirror polishing followed by heat treatment—exhibited a downward trend similar to that of the other samples, albeit at a different solid–liquid contact angle value. The lowest recorded contact angle belonged to sample P2, which was subjected to heat treatment after sandblasting. This sample showed a decreasing trend similar to that of the other analyzed samples.

Therefore, it can be concluded that conditioning metallic samples through sandblasting followed by heat treatment results in hydrophilic behavior, whereas mechanical polishing to a mirror finish leads to hydrophobic behavior. Consequently, hydrophilic materials, which we are looking for, are generally preferred for adhesion and ceramic coatings over hydrophobic ones.

The roughness profile, supported by SEM images, EDS analyses, liquid/solid contact angle measurements, and mechanical tests, demonstrates the advantages of the sandblasting process. This process enhances the bond between the ceramic coating and metallic surfaces. Additionally, it highlights the ceramic coating delaminating issues that can occur when the surface is extremely smooth with a mirror-like finish. Notably, roughness can also be increased in this type of surface through high-temperature treatment, performed an complex oxide layers but not adherent.

Sandblasting the surface with aluminum oxide creates a micro-texture and increases the surface area, allowing for the growth of oxide layers through high-temperature treatments, which leads to the development of a strong hydrophilic effect. Additionally, applying ceramic bonding to the oxide substrate improves the adhesion of dental ceramics to metal frameworks.

In conclusion, this paper can be a guideline for dental laboratory practitioners to know why and how to apply ceramic coatings for the higher adhesion of dental ceramic and their esthetic works.