2023 Volume 64 Issue 4 Pages 869-876
Nickel–Chromium–Molybdenum (Ni–Cr–Mo) alloys are widely used for dental applications. This research was aimed to investigate improvement in the mechanical properties of Ni–Cr–Mo alloy through Boron and Cerium doping. These alloys were doped with boron and boron+cerium alloying elements. The mechanical testing of the samples revealed that minor addition of these alloying elements significantly improved the ultimate tensile strength and yield strength. Moreover, the melting points of the doped samples, determined by differential thermal analysis, decreased appreciably with doping elements. Furthermore, detailed wear testing was carried out to analyze the in-situ behavior of the alloys. Significant improvement in wear rate was noted for the boron and boron+cerium added alloy samples. Also biocompatibility for the three alloys in cytotoxicity test proved the suitability for the use of these alloys in dental prostheses.
Fig. 7 Depth of wear scar variation at different intervals (a): between 100 to 120 seconds, (b): between 1000 to 1020 seconds, (c): between 2000 to 2020 seconds, (d) between 3000 to 3020 seconds, (e): between 3980 to 4000 seconds. Summary of results taken from (a)–(e) is shown in (f).
Dental implants have been in use for over a century. However, dental implants are prone to failure and therefore improvement in performance of the implants for prolonged application is essential for commercial success.1) Dental casting alloys should foremost be nontoxic and biocompatible. They should also exhibit sufficient physical and mechanical properties to fulfill the requisite function and structural stability for prolonged periods of time.2) Alloys used for restorations usually get corroded in oral fluids and produce toxic metal ions at the implant interface.3,4) The most critical features of the oral cavity are teeth that function under very hostile environment of chemical attack, masticating forces, abrasions, attritions, erosions and fatigue, mechanical and thermal stresses. Materials to use for dental restoration must withstand such hostile environment. If the oral environment persists to be acidic, the tooth enamel begins to dissolve, thereby, causing tooth decay. NiCrMo alloy is a base metal dental casting alloy popular for dental restorations due to its overall excellent mechanical and chemical properties and being cost effective.5) Nickel is the main constituent of the alloy and forms austenitic matrix, while chromium the second major constituent provides corrosion resistance.6) Molybdenum addition helps to lower the coefficient of thermal expansion and improves pitting corrosion resistance by formation of oxides on the surface.7) Addition of beryllium can also be made as an alloying element, in Ni–Cr alloys, to enhance casting properties and have improved bond strength with porcelain.8) The formation of Ni–Be eutectic phase in NiCrMo–Be alloys,9) lowers the melting range up to 1160°C and improves castability of the alloy10) and thereby good surface finish of the castings. However, beryllium is known to be a toxic and carcinogenic element and has associated health hazards.11) During melting and fabrication, beryllium in the alloy evaporates causing dental technician exposure to beryllium vapors and dust; which leads to severe damage to lungs, liver and spleen. Commonly used dental casting alloys are summarized in Table 1.
Some advantages of NiCrMo, a non-precious alloy, over other alloys for dental prostheses may be summarized as follows:
NiCr based alloys with chromium content below twenty percent cannot be recommended for application in the oral environment as they release ions and start corroding. Chromium addition enhances passivation of nickel based alloys by formation of a continuous, hard and adherent chromium oxide film.12,13) This oxide layer protects the base alloy from tarnishing and further decay. Titanium and aluminum addition provide strengthening by Ni3Al phase precipitation in austenitic matrix of the alloys.14) These alloys due to lower density, as compared to gold, make them more attractive for dental restorations.15) In developing nations, due to low cost, Ni-based dental alloys are well-liked for dental restorations.16)
Nickel can dissolve Cr and Mo and forms solid solution in the NiCrMo alloys. However, there are some disadvantages of nickel namely; i) cannot be used with nickel sensitive patients, ii) alloys may not cast well, iii) produce more oxides while melting, and iv) polishing is difficult. Ni ion release has associated allergic and toxic effects.17) In order to mitigate theses disadvantages alloying with suitable elements may be utilized to overcome the disadvantages. Cerium addition in various alloys effects to form spherical shaped and small size inclusions and reduces gain size. Also tensile strength, yield strength and elongation are improved.18) Alloying with rare earth metals in previous studies19) indicated that the rare earth metals reduce the oxidation rate and enhance adhesion of oxide layer because of their reactive nature. Corrosion characteristics are not only dependent on chemistry of NiCrMo dental casting alloys but surface oxide characteristics as well. Boron addition presents good thermal stability, improves wear resistance by formation of boro–carbides.20) Boron in various alloys remarkably prevents the alloys from wear.21) Boron alloying to Ni-based dental casting alloys causes to form B2O3 layer at the interface between base metal and the Cr2O3 layer on the surface. Boron is reported22) to form thinner surface oxide films. Furthermore, presence of rare-earth elements such as cerium and lanthanum decreases oxidation kinetics in NiCr alloys and oxide layers formed were reported23) very adhesive on the base alloy.
Friction, wear and abrasion resistance is one of the specific functional requirements for selection of a biomaterial for medical applications.24) These characteristics could be enhanced in dental casting alloys by addition of cerium and boron. Addition of Boron to NiCrMo alloy is reported to increase corrosion resistance and hardness.25) Fracture and wear are more critical factors for patients with bruxing or parafunctional habits. Abrasive wear was reported to be the most frequent reason of damage to restorative materials.26,27) Coefficient of friction ‘µ’ of NiCrMo as reported26) was 0.317 and wear resistance of alloys was reported to be better as compared to stainless steels and Ti-based alloys. As wear causes to decrease implant life, tribological analysis of restoration alloys is an essential requirement to enhance implant life and avoid or delay revision treatments.
Two of the issues faced by dental labs are; high melting temperature and brittleness of the dental alloy. If melting point of Ni–Cr dental alloy is higher, it is usually not appreciated by dental labs, as high melting temperatures need higher power of the furnace and more energy and more time required for melting and casting. While melting and casting at higher temperatures, the quantity of absorbed gases in the molten alloy increases which increase inclusion index and impurities in the alloy and thereby causing brittleness in the alloy. Also recycling of the alloy becomes almost impossible due to inclusion of impurities from the atmosphere. When alloy is in molten state, higher temperature increases the tendency to absorb atmospheric gases like hydrogen, oxygen, nitrogen and carbon that make the alloy brittle.
This research aims to study the NiCrMo alloys and to reduce the melting temperature range through addition of alloying elements and enhance their characteristics without compromising the biocompatibility. Three dental casting alloys were developed and a comparison was made among the three base metal dental casting alloys, namely; i) the conventional NiCrMo alloy, ii) NiCrMo alloy with addition of boron (B), hereafter called NiCrMo–B, and iii) NiCrMo alloy with addition of boron (B) and cerium (Ce), hereafter called NiCrMo–BCe.
The three dental alloys were vacuum induction melted and cast in ceramic shell molds shown in Fig. 1. Pure metals having purity ≥99% were used as raw material. However, ferro–boron was utilized for the addition of boron in the alloy. Rod shaped samples were obtained after casting, Fig. 1(c), which were utilized for subsequent experimentation and characterization. The chemical composition of the alloy samples was determined by using X-ray fluorescence (XRF) and Inductive coupled plasma (ICP) spectrometers. The element boron was measured via ICP and all other elements were measured via XRF. The melting range of the casted samples was measured by utilizing the Differential Thermal Analysis (DTA) using NETZSCH STA449. A 15 mg sample of the alloy was heated up to 1400°C in an alumina crucible with a heating rate of 10°C/min. The heating of the samples was done in an inert argon atmosphere.
Investment casting (a): wax pattern, (b): ceramic shell mold, (c): casting after melting.
Tribological tests were conducted on a Bruker Universal Mechanical Tester Tribolab. Specimens of disc shape with diameter 30 mm × thickness 5 mm were prepared from the neck area of the cast ingots. The test was conducted in normal air environment in ball on disk mode i.e. the hardened steel ball was worn against the horizontal surface of the dental alloy samples. Before this test, the samples were ensured to have same range of surface roughness. The applied load, was kept constant during the test at 5 N. In this test, the sample was rotating at the speed of 48 rpm while total distance run by the ball was 100 meters. This was done in a radial track having a diameter of 10 mm. The calculated velocity during the test was 25 mm/second. During the wear test, the data acquisition rate was kept high and 100 data points were recorded per second. This helped to monitor the in-situ change in coefficient of friction and wear scar depth. Other mechanical properties such as yield strength, ultimate tensile strength and elongation of the samples were measured on Instron Universal Testing Machine by utilizing the standard samples of 100 mm length (diameter at threaded ends was 10 mm and diameter at 32 mm gauge length was 6.25 mm). Impact fracture testing was assessed on Charpy Impact Tester by utilizing the standard sample of 10 mm × 10 mm × 55 mm with v-notch at center; while hardness tests were conducted on CV400 Vickers’s Hardness Testing machine. Phase analysis was carried out using X-ray diffraction on JEOL JDX-8030 machine, with copper kα radiations. Scanning step was 0.05°, scanned from 20 to 90 degree two-theta range. Microstructure was analyzed using EMCRAFTS Cube II scanning electron microscope whereas phase analysis was conducted using attached EDS system. For biocompatibility, cytotoxicity analysis was done against human skin cells (HT144) where the plating density was kept at 1.5 × 105 cells/ml. The test specimens of 0.5 mm thickness × 10 mm diameter were cut by EDM wire cut and chemically cleaned to remove rust and oil from the surface of specimens. Each of the specimens was placed in 6-well tissue culture plate in 1 milliliter of the cell suspension added in each well. The cells were incubated for 24 hours at 37°C. One well of the tissue culture plate, containing HT144 cells with no alloy specimen, was used as the control. The culture plates were removed from the incubator after completion of incubation period and then toxicity of the alloys was assessed.
The chemical composition of the developed alloy samples obtained from two different techniques is summarized in Table 2. It can be seen that the base composition of all the samples has same alloying elements except the addition of about 0.24% B in NiCrMo–B samples and 0.26% B and 0.44% Ce in NiCrMo–CeB samples.
The DTA test results of the three alloys samples are shown in Fig. 2. It can be seen that the samples having no addition of B or Ce, demonstrated higher melting range, as compared to the samples doped with B and Ce. Usually beryllium is added to have dental casting alloys with lower melting temperatures. However, beryllium metal is carcinogenic in nature, and harmful both for dental lab technicians and recipient of the dental implant. Doping of non-precious dental casting alloy NiCrMo with small quantity of boron and cerium helped to lower the melting temperature down to 1220°C as compared with 1160°C for NiCrMoBe alloys. The melting point of NiCrMo and NiCrMoB alloys as measured was 1348°C and 1228°C respectively. The decrease in melting range is helpful in improving the castability of the alloy to enable to fill the intricate shapes during the dental casting process. Lower melting temperature is also preferred by dental labs for ease of fabrication. However, it is noted that the liquidus and solidus temperatures are comparatively narrower in NiCrMo samples than those in NiCrMo–B and NiCrMo–BCe samples due to the formation of eutectic phase.
DTA heating curves showing the melting range of three dental alloys.
As discussed earlier, friction also depends upon the plastic deformation of the material. If a material exhibits more plastic deformation then more friction will be observed. Thus the crystal structures having less number of slip systems such as HCP (hexagonal close-packed) as compared with FCC (faced centered cubic) or BCC (body centered cubic) demonstrate less plastic deformation and therefore lower friction coefficient.28) However, FCC structure provides more ease in fabrication, machining and polishing and therefore, it is preferable over HCP.29) In this study, the results of XRD are depicted in Fig. 3, showing the base alloy NiCrMo is comprised of FCC structure. Other alloys demonstrated similar peaks, since minor amount of B and Ce were doped in the parent alloy.
X-ray diffraction pattern for NiCrMo sample, showing FCC peaks.
Scanning electron micrographs, shown in Fig. 4, of the NiCrMo alloys samples revealed dendritic structure with presence of precipitates. It can be clearly observed that the precipitation increases with alloy additions of B and Ce in the alloys. The EDS analysis demonstrated the formation of molybdenum-silicon precipitates.
Microstructure of the specimens showing the formation of precipitates. (a)–(b): specimen with 0%B, (c): sample with 0.2%B, (d): sample with 0.2%B–0.4%Ce.
The wear behavior of the samples can be assessed by observing the coefficient of the friction (COF). The results of COF for the three samples are presented in Fig. 5. The data is divided into different intervals for the convenience to understand. It can be seen that at the start (Fig. 5(a), for 100 to 120 seconds), the NiCrMo sample demonstrated a large noise where the COF values reached up to 0.8 at some instances. On the other hand, NiCrMo–B and NiCrMo–BCe samples showed minimum amount of COF with almost no noise in the data. When the COF values for the same samples are observed for the intervals between 1000 to 1020 seconds, Fig. 5(b), it can be noted that NiCrMo and NiCrMo–BCe samples showed stick and slip phenomenon. During sliding of one object to another, a stick-slip phenomenon sometimes occurred. This phenomenon occurs between the two surfaces when the coefficient of static friction exceeds than the coefficient of kinetic friction.30) This phenomenon, however, is observed more for NiCrMo samples as compared to NiCrMo–BCe samples. The samples with only boron addition i.e. NiCrMo–B demonstrated no stick and slip phenomenon. With the passage of time, as the ball continues sliding over the plane surfaces of the dental alloy samples, it was observed, Fig. 5(c)–(e), that for the sample NiCrMo, the stick and slip phenomenon relatively decreased between 2000 to 2020 seconds, Fig. 5(c), while once again increase was noted between 3000 to 3020 seconds, Fig. 5(d). This can be attributed to plastic deformation during the sliding which created surface rugosity that was reflected in the form of increased stick and slip phenomenon. However, samples with addition of B and Ce i.e. NiCrMo–BCe, demonstrated a comparatively uniform behavior throughout the test duration, Fig. 5(a)–(e). The samples with only addition of B i.e. NiCrMo–B showed maximum COF with no stick and slip phenomenon. The addition of B in the dental alloy eliminated the stick and slip phenomenon of the alloy, thus reducing the wear rate as well as the coefficient of friction.
Comparison of coefficient of friction (COF) at different intervals of time during the test. (a) between 100 to 120 seconds, (b): 1000 to 1020 seconds, (c): 2000 to 2020 seconds, (d): 3000 to 3020 seconds, (e): 3980 to 4000 seconds.
The atomic size of the B is 85 pm, whereas, that of Ni: 135 pm, Cr: 140 pm, and Mo: 145 pm. Thus B addition is expected to be placed at the interstitial sites in the lattice due to its smaller size. However, the solubility of B is limited in Ni, Cr and Mo.31) Therefore, little hardness increase is expected as demonstrated above. The increase in hardness reduces the plastic deformation and thus comparatively increases the COF with no stick and slip phenomenon. When Ce is added in the sample having NiCrMo and B, the COF behavior is drastically changed due to increase in bulk hardness and formation of cerium oxide on the surface. A very consistent COF observed throughout the wear test, Fig. 5(a)–(e). The metal surfaces which have high surface energies are prone to make a surface layer by reacting with gas and liquid molecules in the atmosphere. This adsorbed layer reduces the surface energy of the metal surface and therefore, reduces the coefficient of friction. In other words, the adhesion of the metal surface will be decreased. Cerium (Ce) is highly oxidizing in nature and therefore, formed a very thin layer of oxide on the surface of the metal. This layer typically 1 to 10 nanometer thick forms very quickly. This oxide layer minimizes the metal-metal contact and therefore decreases the coefficient of friction.
3.4.2 Depth of wear scarDuring the wear test, substantial information is obtained about the depth of the wear scar. The overall data representing the depth of scar against the time is shown in Fig. 6. It can be seen that the sample NiCrMo–B had minimum wear scar depth while NiCrMo sample demonstrated maximum wear scar depth. On observing the data, between close intervals i.e. at the level of 20 seconds, the magnified curve revealed the behavior of the material during the test. These magnified curves are taken for the time intervals of 100 to 120 seconds, Fig. 7(a); 1000 to 1020 seconds, Fig. 7(b); 2000 to 2020 seconds Fig. 7(c); 3000 to 3020 seconds, Fig. 7(d) and 3080 to 4000 seconds Fig. 7(e).
Comparison of the depth of wear scar with wear time.
Depth of wear scar variation at different intervals (a): between 100 to 120 seconds, (b): between 1000 to 1020 seconds, (c): between 2000 to 2020 seconds, (d) between 3000 to 3020 seconds, (e): between 3980 to 4000 seconds. Summary of results taken from (a)–(e) is shown in (f).
Each magnified curve demonstrates the spring back effect during the wear test. It can be seen in these curves that on the application of applied load, which is in the elastic limit, the underlying material deformed elastically and then spring back to its original position. However, due to repeated applied loading and unloading a change in material was observed which is plotted in Fig. 7(f). In this plot, the data is taken from the curves shown in Fig. 7(a)–(e). The values obtained, from the difference of trough and crest, were plotted and summarized in Fig. 7(f). It can be seen that NiCrMo sample showed maximum change i.e. the spring back effect decreases drastically as compared to the other two samples. The decrease in spring-back effect referred to the fact that the material is plastically deformed or material is fatigued under the constant loading and unloading conditions in elastic region. Further, the other two samples having B (NiCrMo–B) and B+Ce (NiCrMo–BCe) additions behave differently in the same loading and unloading conditions. The NiCrMo–B sample seems stable with no appreciable change in the spring-back effect (Fig. 7(f)). However, the NiCrMo–BCe sample demonstrated that the spring-back effect increased with the passage of time. This shows that the material is work hardening during the course of the wear test.
3.4.3 Wear rateOptical micrographs of wear scar, comparing the width, are shown in Fig. 8. The difference in wear scar can be noted for the three alloys. Wear rate of the samples was calculated using the following relationship:
| \begin{equation*} \text{Wear rate} = \frac{\text{V}}{\text{F} \times \text{L}} \end{equation*} |
Optical micrographs demonstrates the width of wear scar for: (a) NiCrMo, (b) NiCrMo–B, and (c) NiCrMo–BCe samples.
Comparison of wear rate for the three dental casting alloys.
It can be seen from Table 3 that the samples having B+Ce addition exhibit maximum UTS i.e. 513 MPa as compared with 394 MPa and 312 MPa for NiCrMo–B and NiCrMo samples respectively. Other mechanical properties i.e. yield strength; percentage-elongation, hardness and impact toughness are also mentioned in Table 3. Most of the fractures were reported32,33) to occur due to fatigue and impact, therefore, impact strength is one of the important material characteristic to be considered while developing or selecting a material for dental restoration. The addition of B and B+Ce in NiCrMo alloy improved the yield and hardness of the alloy, whereas, impact toughness slightly compromised with the addition of both B+Ce i.e. in sample NiCrMo–BCe. In general, hardness was found to be inversely proportional to the coefficient of friction. This is due to the fact that harder surfaces provide more resistance to plastic deformation. However, hardness of the alloy should be sufficient enough to be wear resistant by the opposing tooth and it should not be high enough to cause excessive wear of the opposing teeth enamel. It can be noted from Table 3, that after addition of B (NiCrMo–B) or B and Ce (NiCrMo–BCe), the hardness did not increase excessively. Yield strength and hardness for Be-containing NiCrMo dental casting alloy is 325 MPa and 350 Hv respectively as reported by Wataha.34)
Biocompatibility test of NiCrMo alloys was performed to check suitability for dental prostheses. The samples were tested for biocompatibility against human skin cells (HT144) with platting density of 1.5 × 105 cells per milliliter for 24 hours exposure at 37°C. The growth of the cells results after 24 hours incubation is shown in Fig. 10. All the three alloys were found to be biocompatible, as the pattern of cells was almost same as that observed in the well of the control cells. Least or no cytotoxicity in NiCrMo alloys may be related to the high contents of chromium and molybdenum; which develop an adequate protective oxide layer on the surface and make the alloy more corrosion resistant. It can be inferred that cytotoxicity of the NiCrMo dental casting alloys depends upon the amount Cr and Mo in the structures. While comparing beryllium free NiCrMo alloys with the Be-containing NiCrMo dental casting alloys it was reported35) that the Be-containing alloys release more Be and Ni ions that have associated health hazards for continued use of these alloys.
Cytotoxicity test graphical results for human skin cells (HT144); (A) unexposed cells culture sample, and cells exposed to; (B) NiCrMo base alloy, (C) NiCrMo–B alloy, (D) NiCrMo–BCe alloy samples.
The cells were observed for changes in morphology, reduction of cell growth, cell lysis and detachment from surface. No significant change in the mentioned parameters was observed for the three alloys investigated (Fig. 10(B), (C), (D)). Cells were attached with the surface and overall culture health was same as unexposed culture (Fig. 10(A)). This can be attributed to the fact that the three NiCrMo alloys investigated are potentially non-toxic.
NiCrMo dental alloy was analyzed and compared with boron and cerium doped samples. The characterization of the three samples revealed that face centered cubic structure formed in all the samples, which offer ease in fabrication. The wear testing of the samples show that the sample doped with boron and cerium had maximum wear resistance. This is due to the formation of cerium oxide layer on the surface of the samples. It is also demonstrated that NiCoMo–BCe samples constantly work-hardened with time. Mechanical properties of the same alloys also revealed that doping of minor amount of boron or boron and cerium helped to improve the mechanical properties. All the samples showed good bio-compatibility and had no toxic effect. Also in comparison with the commercially used NiCrMo–Be alloy, the B and Ce doped samples showed almost similar melting temperature range and strength while offering better biocompatibility.
The authors acknowledge the cooperation of “Institute of Biomedical and Genetic Engineering”, Islamabad for helping in testing the samples for biocompatibility.
