Evaluation of Topographical Co-Cr-Mo Alloy Surface Changes After Various Finishing Treatments.

PURPOSE: To quantify the influence of three different finishing treatments on the cobalt-chromium-molybdenum (Co-Cr-Mo) alloy surface based on stereometric analysis parameters.
MATERIALS AND METHODS: Eighteen specimens were casted from an extra-hard alloy (Wironit®, BEGO, Bremen, Germany). The samples were distributed into three groups (n = 6 samples per group) dependent on different polishing techniques applied, as follows: A group, only electropolished (EP) samples; B group, after EP, an additional mechanical polishing process was applied to the surface by rubber discs and a polishing paste (RP); C group, after EP, an additional mechanical polishing process was completed by rubber discs, polishing paste and finally by a rotating deer leather wheel (RPDL). Samples were imaged by atomic force microscopy (AFM) in a contact mode, in air, at room temperature.
RESULTS: The evaluation of the microtexture of the sample surface was made based on the 3-D roughness parameters. The lowest statistical surface roughness parameters were found in the RP samples, whereas the highest values were obtained from the EP samples.
CONCLUSIONS: The experiments described can help manufacturers identify the most appropriate parameters and their ranges within which optimal surface characteristics can be achieved.

Introduction

In the last few decades, cobalt-chromium-molybdenum (Co-Cr-Mo) alloys have shown high mechanical features and resistance to wear and corrosion when used as orthopedic implant materials or in other biomedical applications. These alloys belong to a group of the hardest known alloys with biocompatible characteristics. They also possess some other good properties such as tensile strength, fatigue resistance, as well as good corrosion resistance. Co-Cr-Mo alloys are now commonly used as implants in artificial hip and knee joints, and in the manufacturing of metal frameworks for removable dentures in order to prevent fractures. In advanced biomaterials studies, most of them deal with biological response mechanisms to carry residues (metal particles or metal oxide particles, etc.) and oxidation products (cations, metal oxides, metal phosphates, etc.).

In contemporary dentistry, Cobalt-Chromium (Co-Cr) dental alloys have been utilized lately in applications which require highly biocompatible, wear resistance, corrosion resistance and/or thermal resistance features (-). Removable partial dentures (RPDs) have been manufactured mostly of this alloy, although the alloy can also be utilized for fixed partial dentures. Traditional fabrication technique for Co-Cr-Mo alloy is casting.

Additive manufacturing (AM) technology leads to exciting new developments in the field of medical implant manufacturing. Selective laser melting (SLM) is an additive cutting edge technology that can be applied in the production of complex metal parts, as well as for the manufacture of complete or partial removable and fixed prostheses. The realization of materials with advanced properties can be achieved by the SLM casting process which is essentially based on the rapid melting and rapid solidification of the metal powder (). A high-energy laser beam acts on the metal powder as a heat source and causes rapid melting, followed by rapid solidification over a short period of time. The rate of melting and solidification depend on the energy density of the laser. However, SLM produced alloys need additional research in terms of long-lasting biocompatibility and technical precision. Due to a lack of long lasting evidence and high price of new technologies, most of dental laboratories worldwide still use traditional procedures for metal framework production such as casting.

Surface characteristics of a dental alloy are in direct correlation with the area of the contact surface. These characteristics influence its biocompatibility, its corrosion, and microorganism and/or how it interacts with human cells ().

Some studies found out a correlation between the high value of parameters of surface roughness of materials in the oral cavity and increased incidence of oral infectious diseases, such as dental caries, gingivitis, periodontal disease, and inflammation of the oral mucosa (-). Mechanical techniques that smoothed surfaces for application in the oral cavity showed positive influence on both, oral health and resistance to corrosion of an alloy, moreover, bacterial adhesion and colonization was decreased ().

Several researchers highlighted that surfaces of Co-Cr dental alloys appear in a form of typical dendritic microstructure after solidification. Under a light microscope the surface of the alloy showed dark regions which were alternating with light regions (, ).

The evaluation of surface micromorphology of Co-Cr-Mo dental alloy with AFM studies (), in correlation with the manufacturing process (), has been made in recent years (-). On the other hand, some investigators applied modern mathematical tools (-) for the 3-D microtexture description of various dental materials (-).

In this study, the surface microtexure of the Co-Cr-Mo dental alloy subjected to different sequences of electropolishing and further mechanical polishing procedures was studied, using AFM stereometric analysis.

Materials and methods

Samples

A commercial Co-Cr-Mo alloy (Wironit® extra-hard, Bego, Germany, 63% Co, 30% Cr, 5% Mo, and Si, Mn and C < 2%) was used for sample preparations. A total of 18 casted samples were made and finally polished in three different ways. Square wax samples 8x8x2 mm were made from wax (Modelling wax standard, DeguDent GmbH, Germany) and were invested into the investment material (Castorit super C; Dentaurum, Ispringen, Germany) as recommended by the manufacturer.

We preheated the wax samples at a recommended temperature (1273 K) and then we casted them with a vacuum casting machine (Nautilus, Bego, Germany) at a recommended temperature (1693 K), according to the manufacturer recommendations. After casting and cooling, we cleaned off the investment material particles in a sandblasting machine (Austenal Dentastrahl VE2, Germany) with particles of aluminum oxide (250 µm), until all residues of the investment material were removed.

Final surface treatments

Sandblasting and a 15-minute electro polishing were applied to all samples (Eltropol SL electropolishing machine, Bego, Germany). Megalyt Megadental GmbH, Germany was the electrolyte polishing solution. Subsequently, those specimens were distributed into groups A, B and C (each one consisting of six specimens).

The A group comprised 6 electropolished specimens (EP) without any further manufacturing.

The B group (RP), comprised six electropolished samples, which were additionally polished mechanically by a rotating rubber disk polisher (Fine-grit, Dentaurum, Ispringen, Germany) and finally finished using a polishing paste for a high shine (Sherapol 705, Shera GmbH, Germany) and a rotating polishing brush.

The C group, (six RPDL samples), was first electropolished and, further polished mechanically with a rotating rubber disk (Fine-grit, Dentaurum, Ispringen, Germany) and a polishing paste (Sherapol 705, Shera GmbH, Germany). Finally, the RPDL sample polishing was completed with a 70-mm radius deer-leather rotating polishing wheel (Dlesk, Zagreb, Croatia). The deer leather polishing wheel and the paste were used for three minutes.

The mechanical polishing procedures (after the electropolishing procedure) in the group two and the group three were done under light pressure by one experienced dental technician. The dental technician tried to equalize the pressure of specimens as much as possible against the rotating polisher.

Sample cleaning

To be ready for the AFM analysis, specimens were thoroughly cleaned. The following procedures were performed on all samples: cleaning with alcohol; rinsing in ultrapure water; sonicating for a period of 10 min in ultrapure water; drying with absorbent paper.

AFM analysis

We obtained AFM micrographs (100 μm × 100 μm) of the specimens at constant temperature, air humidity, in air, in a contact mode, by the Multimode AFM, and the Nanoscope IIIa controller. Surface regions of interest (ROIs) were identified by an optical camera (Sony high resolution CCD camera, Japan). A more detailed description of the AFM measurements can be found elsewhere (-).

3-D surface roughness statistical description

MountainsMap® Premium software version 7 was applied to extract information about the 3-D surface microtexture (). These parameters of 3-D surface microtexture are described in ISO 25178-2:2012 ().

The computation of Minkowski functionals (MFs) (volume V, surface S and Euler-Poincaré characteristics (or connectivity number χ)) was made with Gwyddion 2.37 software () based on the following mathematical formulas:

where: N is the total number of pixels; N is the number of ‘white’ pixels, which are pixels above the threshold; N is the number of white-black pixel boundaries; and C and C are the number of continuous sets of white and black pixels, respectively.

Statistical analysis

Statistical Package for the Social Sciences (SPSS, version 20, Illinois, USA) was used to analyze statistics. Quantitative variables between different groups were compared by one-way analysis of variance (ANOVA), and Post hoc test by the Scheffé test. The level of significance was considered significant for p < 0.05.

Results

The relevant 2-D AFM images of samples, for a 100 x 100-μm2 scanning square area, are shown in Figure 1, for all three groups.

Figure 1

The relevant 2-D topographic AFM micrographs, for a 100 x 100-μm2 Co-Cr-Mo dental alloy surface scanning square area: a) EP sample; b) RP sample; c) RPDL sample.

Figure 2 also shows a set of Sk parameters. Parameter Sk provides information on the depth of the core and can be determined using a line that intersects the X-axis on 0% and 100%. On the graphical representation, two lines parallel to the X-axis are drawn and the computed values of parameter Sk for each group are: a) Sk = 0.17 μm; b) Sk = 0.201 μm; c) Sk = 0.183 μm.

Figure 2

Areal material ratio curve with parameters: Sk, Spk, Svk, Smr1 and Smr2 for each group of the Co-Cr-Mo dental alloy: a) EP sample; b) RP sample; c) RPDL sample.

To graphically determine the values ​​of the parameters, Smr1 and Smr2 are drawn parallel lines with the Y-axis, which will intersect with the parallel lines with the X-axis, previously drawn in the calculation of the parameter Sk. The computed values of Smr1 parameter are: a) 13.8%, (Figure 2a), b) 11.8%, (Figure 2b); c) 11.7%, (Figure 2c), while for Smr2 parameters are: a) 88.6%, (Figure 2a), b) 89.8%, (Figure 2b), c) 85.7%, (Figure 2c).

Computation of Svk and Spk parameters is performed from the height of the perpendicular triangles represented on the diagram (with an orange color): a) Svk = 0.0972 μm and Spk = 0.128 μm; b) Svk = 0.0549 μm and Spk = 0.0929 μm; c) Svk = 0.0982 μm and Spk = 0.104 μm.

From the category of the functional parameters for the volume are calculated Vmp, Vmc, Vvci, Vvv (Fig. 3), considering an analogous calculation method, as well as the data obtained in the area material ratio curve. The calculation method takes into account the value of the parameters mr1 (10%) and mr2 (80%). The following values were obtained for peak material volume (Vmp): a) 0.00676 ml/m2, b) 0.00472 ml/m2, c) 0.00508 ml/m2, as well as for the core material volume (Vmc): a) Vmc = 0.0628 ml/m2, b) Vmc = 0.0744 ml/m2, c) Vmc = 0.071 ml/m2. For dale void volume (Vvv) were obtained the following values: a) 0.00995 ml/m2, b) 0.00715 ml/m2, c) 0.0111 ml/m2, while for core void volume (Vvc) these are: a) 0.097 ml/m2, b) 0.105 ml/m2, c) 0.0981 ml/m2.

Figure 3

Areal material ratio curve with parameters: Vmp, Vmc, Vvc and Vvv for each group of the Co-Cr-Mo dental alloy: a) EP sample; b) RP sample; c) RPDL sample.

The peak count histograms from Figure 4, a-c were computed according Ref. ().

Figure 4

The peak count histograms for each group of the Co-Cr-Mo dental alloy: a) EP sample; b) RP sample; c) RPDL sample

The computed MFs functions V(z), S(z), and X(z) are shown in Figs 5-7.

Figure 5

The Minkowski volume V(z) [no unit] for each group of the Co-Cr-Mo dental alloy: a) EP sample; b) RP sample; c) RPDL sample.

Figure 6

The Minkowski surface S(z) [no unit] for each group of the Co-Cr-Mo dental alloy: a) EP sample; b) RP sample; c) RPDL sample.

Figure 7

The Minkowski connectivity X(z) [no unit] for each group of the Co-Cr-Mo dental alloy: a) EP sample; b) RP sample; c) RPDL sample.

In Figures 8 and 9, the autocorrelation function (ACF), computed with a model of linear interpolation type: a) in a horizontal direction; b) in a vertical direction, are graphically represented

Figure 8

The autocorrelation function (ACF), based on the linear interpolation type, for each group of the Co-Cr-Mo dental alloy: a) EP sample; b) RP sample; c) RPDL sample in horizontal direction.

Figure 9

The autocorrelation function (ACF), based on the linear interpolation type, for each group of the Co-Cr-Mo dental alloy: a) EP sample; b) RP sample; c) RPDL sample in vertical direction.

Discussion

In the biomedical field, in the last few decades, a series of metal alloys have been successfully used as biomaterials. Chemical composition of both, cast and selective laser melting manufactured alloys are standardized, however, the microstructure of an alloy can vary in a size of its grains and phase types. In casted alloys, the microstructure depends on the process of manufacturing, heat treatment, cooling and final finishing (polishing). Therefore, we aimed to find out whether the surface of cast Co-Cr-Mo alloy varies dependent on certain polishing procedures which are the most frequently performed in dental laboratories because the difference in a surface microtexture may influence the alloy’s corrosion behavior.

Some researchers (, ) reported that cast Co-Cr-Mo alloys have dendritic microstructured surface with 1 to 4-nm thick stable oxide layer that constitutes a biological corrosion barrier opposing oral environment (). Additionally, 20 to 40-µm long and 150 to 400-µm high undulations of the smooth surface have been reported ().

An undulating pattern can be observed on the EP samples with islands of crystallites emerging from the surface with a typical saw-tooth pattern on the top (Figure 1a).

The surface microtexture of a RP sample is shown in Figure 1b where “islands” preserve the contour (perimeter) similarly to EP samples, but the peaks of crystallites are smoothed (removed), as a result of additional polishing. Crystallites forming islands have been smoothed and polished to the height of 40 nm. As a result of the action of a rotary brush, on the AFM microphotographs, one can observe parallel scratches drawn and on the undulating surface of the specimens. These scratches are V-profile 13-26 µm apart from each other. Their surface openings measure approximately 0.5-2 µm in width and are 15-100 nm deep.

The RPDL sample surface microtexture is shown in Figure 1c. On these samples, the undulating surface has more scratches than RP samples; the distances between scratches are smaller (denser scratches). The undulated surface and islands of smoothed crystallites are crossed by many parallel scratches.

Computed values of the statistical surface roughness parameters, in accordance with ISO 25178-2: 2012 () are shown in Table 1.

Table 1

The computed values of the statistical surface roughness parameters for each group: a) EP sample; b) RP sample; c) RPDL sample.

The statistical parameters	Symbol	EP sample	RP sample	RPDL sample	P values *
		Values	Values	Values	[-]
Sk parameter	Sk [μm]	0.17	0.201	0.183	0.012
Smr1 parameter	Smr1 [%]	13.8	11.8	11.7	0.016
Smr2 parameter	Smr2 [%]	88.6	89.8	85.7	0.014
Svk parameter	Svk [μm]	0.0972	0.0549	0.0982	0.027
Spk parameter	Spk [μm]	0.128	0.0929	0.104	0.025
Peak material volume	Vmp [ml/m2]	0.00676	0.00472	0.00508	0.021
Core material volume	Vmc [ml/m2]	0.0628	0.0744	0.071	0.028
Dale void volume	Vvv [ml/m2]	0.00995	0.00715	0.0111	0.029
Core void volume	Vvc [ml/m2]	0.097	0.105	0.0981	0.018
Arithmetic mean height	Sa [μm]	0.0662	0.0638	0.0643	0.019
Root mean square height	Sq [μm]	0.086	0.0816	0.0847	0.022

* Statistically significant difference for all values: P < 0.05.

Table 1

The smallest arithmetic mean height Sa (0.0638 μm) was obtained from the RP samples, while the highest value (0.0662 μm) was obtained on the EP samples. The mean height Sa of 0.0643 μm was observed for PRDL samples. Similar ordering was found for the root mean square height Sq parameter.

The highest values of: Sq, Sa, Vm, Vmp were recorded on the EP specimens. The RPDL specimens had higher values of Sq, Sa, Vm, Vmp, and Vvv than the RP samples. The RP specimens had the smallest values, respectively.

Topographical results obtained from AFM are confirmed by the Minkowski functionals (MFs) and the autocorrelation functions (ACF) in horizontal and vertical direction.

After the EP procedure, mechanical polishing changed the morphology of the islands of crystallites emerging vertically from the undulating surface. As the crystallite peaks of islands of EP samples were smoothed by additional mechanical procedures, surface roughness generally decreased. However, the scratches formed by further polishing procedure were mostly parallel to each other. The surfaces of islands became almost featureless (without any peaks) and were smooth on the top. The RPDL samples appeared to have more scratches than the RP samples; the depths of RPDL scratches were up to 100 nm, thus increasing surface roughness; therefore, the smoothening of islands did not cause a significant decrease of overall surface roughness (Figure 1b and Figure 1c). However, deeper scratches, formed by mechanical polishing and consequent crystal grain deformation in the RPDL samples may lead to intergranular corrosion and crystallographic etching in intraoral aggressive environment.

The obtained results of this research are in line with studies (, ), which demonstrated that the surface microtexture can be correlated with the composition of dental materials and the corresponding protocols of surface polishing.

Conclusion

This study suggests that in the case of final surface finishing procedures, the specific micromorphology parameters of the surface microtexture on different surfaces of Co-Cr-Mo alloy are influenced by the polishing effects. These statistical surface roughness parameters are important in designing and manufacturing materials with optimal texture surfaces.

Appendix

The definitions of main statistical parameters applied in 3-D surface roughness analyses (height parameters, functional parameters and functional volume parameters), defined in accordance with ISO 25178-2: 2012 () are presented in Figure 10.