1067 D3000 new imprint Word template Vol 13, No 1 (2025) ISSN 2167-8677 (online) DOI 10.5195/d3000.2025.1067 http://dentistry3000.pitt.edu 3D Printed Co-Cr Alloy Surface Treatments Effect on Shear Strength of Heat Cured Resins Luay Abdullah Khalaf, Luma Mudhafar Al-Nema College of Den*stry, University of Mosul, Mosul, Iraq Abstract Objec7ve: To investigate the effect of different surface treatments—hydrochloric acid etching, sandblasting, metal primer application, and a combination of sandblasting with metal primer on the shear bond strength (SBS) of heat-cured polymethyl methacrylate (PMMA) bonded to 3D-printed Co-Cr alloy, and to assess the failure modes. Materials and Methods: Fifty disk-shaped 3D-printed Co-Cr specimens were divided into Give groups (n=10 each): control (C), acid etch (A), sandblast (S), metal primer (P), and sandblast + primer (SP). Specimens were bonded to heat-cured PMMA resin and tested for shear bond strength using a universal testing machine. Failure modes were examined micro- scopically. Data were analyzed using one-way ANOVA and Duncan’s multiple range test. Results: SigniGicant differences in SBS were observed among groups (p ≤ 0.05). The SP group showed the highest SBS (15.78 ± 1.19 MPa), followed by P (12.77 ± 2.61 MPa) and S (5.80 ± 1.01 MPa). The A (0.43 ± 0.08 MPa) and C (0.37 ± 0.07 MPa) groups exhibited the lowest SBS values. Failure mode analysis revealed adhesive failures in C, A, and S groups, while P and SP showed predominantly mixed failures. Sandblasting increased surface roughness, while primers enhanced chemical bonding without altering topogra- phy. Conclusions: The combination of sandblasting and metal primer achieved the highest bond strength, conGirming a synergistic effect between microme- chanical and chemical bonding. Acid etching alone was ineffective. Com- bined treatments are recommended to improve the clinical performance of 3D-printed Co-Cr RPD frameworks. Open Access Cita%on: Khalaf LA, et al. (2025) 3D Printed Co-Cr Alloy Surface Treatments Effect on Shear of Heat Cured Res- ins. Den%stry 3000. 1:a001 doi:10.5195/d3000.2025.1067 Received: October 8, 2025 Accepted: October 15, 2025 Published: November 10, 2025 Copyright: ©2025 Khalaf LA, et al. This is an open access ar%cle licensed under a Crea%ve Commons AVribu%on Work 4.0 United States License. Email: Luma2005@uomosul.edu.iq Introduc)on A removable parEal denture (RPD) is an ac- cepted device for treaEng paEents with parEal tooth loss. They help replace missing teeth and improve funcEonality. RPD usually consists of a base that supports the arEficial teeth, along with a metal framework that provides struc- tural integrity and stability. This combinaEon ensures proper fit and funcEon, enhancing the wearer's ability to chew and speak effecEvely. Cobalt-chromium (Co-Cr) alloy is oWen chosen for the framework due to its favorable proper- Ees, which include cost-effecEveness, elevated mechanical strength, excellent resistance to corrosion, and suitability for manufacturing processes [1,2]. Polymethyl methacrylate (PMMA) is com- monly uElized as a denture base material for RPD due to Essue compaEbility, aestheEc qual- iEes, and favorable material characterisEcs [3,4]. AddiEve manufacturing (AM) technology, of- ten referred to as three-dimensional (3D) prinEng technology, has increasingly been uE- lized to produce various dental prostheses [5,6]. With addiEve AM, we can create the metal framework using materials that are sur- prisingly like those tradiEonally used, like Co-Cr powder. This innovaEve approach opens up ex- ciEng possibiliEes for enhancing design and funcEonality [6,7]. Any flaws or separaEons occurring between the acrylic resin and the metal framework, es- pecially along the finishing line, may lead to cracks or crazing in the acrylic resin. As a result, a weak bond can have a direct effect on the juncEon between the metal and the resin [4]. TradiEonally, metal frameworks are secured to denture base resins using mechanical reten- Eon techniques, which can involve loops, meshes, beads, nail heads, and undercut finish lines. Shear strength between the acrylic resin base and metal differs depending on the type of mi- nor connector (labcework or mesh) [8]. The applicaEon of tradiEonal mechanical re- tenEon methods for bonding metal to resin has 3D Printed Co-Cr Alloy Surface Treatments Effect on Shear Strength of Heat Cured Resins Vol 13, No 1 (2025) DOI 10.5195/d3000.2025.1067 http://dentistry3000.pitt.edu 2 been extensively invesEgated, yet it fails to eliminate microleakage [9]. Deficiencies in the chemical bond between metal and acrylic resin can lead to significant clinical issues, including microleakage of oral fluids. Such complicaEons may be miEgated through the establishment of a robust chemi- cal bond [10]. The chemical bonding of the denture base resin to the metal is preferred over mechanical retenEon in construcEng RPD. Metal primers present a straigheorward approach. They are a more economical choice relaEve to tradiEonal methods. Special equipment is not necessary, and the applicaEon process is not parEcularly sensiEve to technique [10,11]. This study was designed to evaluate the effect of various surface treatments on shear bond strength of heat polymerized acrylic resin bonded to 3D printed Co-Cr alloy. The null hypothesis was that the shear bond strength between the Co-Cr alloy and 3D printed Co-Cr alloy would not differ, regardless of the types of various surface treatments. Materials and Methods Study design In this study, 50 specimens were disk-shaped and used for the shear bond strength test. The specimens were divided into five groups based on the surface treatment performed on the specimens, each group had 10 specimens and as follows: Group C: This is the control group in which no surface treatment was done on the bonding surface of the specimens. Group A: The bonding surface of alloy samples in this group was treated with hydrochloric acid (10%). Group S: The bonding surface of alloy samples in this group was sandblasted with110 μm alu- minum oxide parEcles. Group P: The bonding surface of alloy samples in this group was treated with metal primer. Group SP: The bonding surface of alloy samples was sandblasted with 110 μm aluminum oxide parEcles, and then metal primer was applied. Specimen Design SketchUp (Trimble, USA) version 2024 was used to design the geometrical shape of the specimens. The specimens prepared for the as- sessment of shear bond strength were fabri- cated as disk-shaped specimens with a 10 mm diameter and 2 mm height [11-13]. All the de- signs were exported in an STL format. Specimen prepara7on The specimens were fabricated by a selecEve laser melEng (SLM) machine (D-150, Riton, China), First, the STL files of the specimens' de- sign were imported into the SLM soWware sys- tem and mulEplied to the required number of specimens. All specimens were distributed vir- tually in soWware within the boundaries of the stainless-steel plate to be printed. The samples were arranged on the plaeorm so that their 3-mm wide sides were aligned with the z-axis, and the SLM was performed in a ni- trogen environment with a laser energy of 10 kilowals and an energy density of 1.964 J/mm2. The laser power (P) determined the energy density of 165W. The building direcEon was 90 degrees, the speed of the scan was 1050 (mm/s), and the scan line spacing was S. (mm). The parEcle size of the Co-Cr powder alloy (RITON, China) was between 12 and 65 mi- crons, and the layer thickness was fixed to 25 microns [14]. The Co-Cr powder was applied to the stainless- steel plate. The machine started melEng the powder layer by layer unEl the samples were fully constructed according to the manufactur- er's guidelines. When specimens’ fabricaEon is finished, re- moving the support is easily done through the band saw (DLY-1BF1, Riton, China). Finishing of the specimens was carried out with diamond grinding stones, then all the speci- mens were finished by using the abrasive wheel to remove any irregulariEes from the margin of the specimen [15,16]. For gebng a good polished specimens surfaces a grinder polisher device (MP-160E,GOYOJO, Hong Kong) was used, the abrasive silicon car- bide paper number 600 was used under run- ning water for 10 seconds and the grinder speed was set to 300 rpm to provide a flat and uniform surface [11,12,17-19]. Applica7on of Heat-Cured Acrylic Resin To facilitate the fabricaEon of the acrylic wax palern, a special split mold was made from stainless steel by a CNC machine to produce a wax palern, a disk shape with 5 mm in diame- ter and 2 mm in thickness. The mold was fabri- cated to accommodate the Co-Cr disk with a di- mension of 10mm in diameter and 2mm in thickness. Above the disk specimen, the mold would be narrowed in diameter up to 5 mm and extended 2 mm. This created a space of 5mm in diameter and 2mm in thickness in the center of the disk specimens, as displayed in Figure 1, where modeling wax (NK, China) was applied [11,12]. The final shape of specimen aWer applicaEon of molten wax is shown in Figure 2. These specimens (Co/Cr alloy disk modeling wax assemblies) were flasked in a standard flasking technique for acrylic dentures with dental stone (Singletypo3,Lascod, Italy) and then the specimens were dewaxed and cleaned using boiled water [4]. Before the packing procedure, the specimens were divided into five groups, each with ten specimens, and then labelled according to the surface treatment they received. For control group, the Co-Cr specimens with- out surface treatment. For acid etch group, apply the hydrochloric acid 10% (Chemlab Hydrochloric Acid, AnalyEChem, Belgium) over the bonding surface of the Co-Cr specimens. AWer 30 minutes, the specimens were rinsed with deionized water for 30 sec- onds [20]. For air abrasive group, the specimens were abraded with aluminum oxide 110µm (Cobra Abrasive, Renfert, Germany) using a sandblast- ing machine (Sandblaster T1Mestra, Spain), at 4 bar pressure, and carried out for 14 seconds. The distance between the nozzle Ep and the surface of the specimens was maintained at 2 cm, and held perpendicular to the Ep [11]. For Metal Primer Group, 2- 3 drops of the metal primer (METAL PRIMER Z, GC; Japan) were dispensed into a clean dispensing dish, and then, using a brush, a thin layer was ap- plied to the bonding surface and allowed to dry for 5 seconds. Immediately aWer that, the heat- cure acrylic was applied to the treated surface [11]. For specimens , to be treated with both sand- blasEng and metal primer, sandblasEng was followed by applicaEon of metal primer [11]. AWer the surface treatment of samples, The heat cured acrylic resin(SR Triplex Hot, Ivoclar, Liechtenstein) mixed with powder: liquid raEo of 3:1 and packed by placing the flask in a hy- draulic press (Flask Press, Quayle Dental, UK), with an applied pressure of 2000 psi, and then clamped for curing [21]. The clamped flasks were then placed in a water bath; the water was heated gradually from room temperature for about 45 minutes to a boiling temperature of 100Сo and maintained at this temperature for 30 minutes according to the manufacturer's instrucEons. The flask was allowed to bench cool slowly at room tem- perature [15]. The specimens were then deflasked and cleaned by using an ultrasonic cleaner(Elec- tronic Scale) for 20 minutes, and then stored in disElled water(Al-Joud, Iraq) for 24 hours ac- cording to ADA specificaEon number 12 [22]. Shear Bond Strength Test The specimens inserted in the center of auto polymerizing acrylic resin base (Dentway Self- Cure, Dentway, Turkey) by using a special sili- con mold. To obtain the bond strength. The specimens were mounted in the universal test- ing machine (HSA-UT, Dongguan Hongjin Test Instruments (DHTI), China) with the help of the specimen holder in a way that the treated specimen's surface was parallel to the loading piston, shown in Figure 3. The loading piston had a chisel configuraEon, which was applied with a crosshead speed of 3D Printed Co-Cr Alloy Surface Treatments Effect on Shear Strength of Heat Cured Resins Vol 13, No 1 (2025) DOI 10.5195/d3000.2025.1067 http://dentistry3000.pitt.edu 3 0.5mm/min [11].The maximum load required at failure was recorded, and the shear bond strength was calculated by dividing the load by the surface area using the following equaEon: SBS (MPa) = F (N)/ S (mm2). SBS = shear bond strength (MPa), F = the load at failure (N), and S = surface area (mm2). The load is the value at which the specimen deboned, and the surface area is the bonding area of the circular specimen that was 5 mm in diameter. The surface area was calculated ac- cording to the equaEon (s)=πr2, where (r) rep- resents the radius of the bonding area [23]. Following the shear bond strength test, the Types of failures were observed under an opE- cal microscope (Hi-Scope; Hirox Co. Ltd.) at 10 X magnificaEon. The mode of failure was cate- gorized as adhesive failure if it took place at the interface between the resin and the metal, or cohesive failure if the separaEon occurred within either the metal or the resin. In in- stances where both types of failure were noted, the failure mode was classified as mixed-type failures [21,24,25]. Sta7s7cal Analysis The Shapiro-Wilk normality test was first done. Mean, standard deviaEon, analysis of variance and Duncan's mulEple rang test were carried out as staEsEcal analysis for data using a staEs- Ecal soWware program (SPSS version 25.0; IBM Corp, USA). The level of significance was set to α=0.05. Results All groups saEsfied the normality assumpEon (all P > 0.05), and therefore one-way ANOVA was performed. Table 1 shows descripEve staEsEcs, including the mean values, standard deviaEon, standard error, and extreme values (minimum and max- imum). The unit of measurement used is MPa. StaEsEcally, the lowest mean values were found in the Control group (0.369 ± 0.074 MPa) and then the Acid group (0.433 ± 0.081 MPa). The Sandblast group had a significantly higher mean value of (5.797 ± 1.011 MPa), while the Primer group had a staEsEcally higher mean value of (12.769 ± 2.614 MPa). The combina- Eon group of sandblast and metal primer had the highest mean value (15.783 ± 1.193 MPa) (Table 1). One-way ANOVA test was performed to deter- mine if there was a significant difference be- tween any of the studied groups. one-way ANOVA had a significance value of 0.000, so there was a significant difference between at least two groups, as shown in Table 2. Duncan’s mulEple range test was performed, which showed that there was no significant differ- ence between only the Control and the Acid groups. However, there was a highly significant difference between all the other groups, as shown in Figure 4. Duncan’s mulEple range test was performed, which showed that there was no significant dif- ference between only the Control and the Acid groups. However, there was a highly significant difference between all the other groups, as shown in Figure 4. Failure Mode Evalua7on The control (C), acid (A), and sandblast (S) groups all exhibited 100% adhesive failures, with no instances of cohesive or mixed failure observed. The metal Primer (P) group showed a clear shiW in failure behavior: 60% of the sam- ples exhibited mixed failures, while only 40% were adhesive. In the CombinaEon group sand- blast and metal Primer (SP), failure mode anal- ysis revealed 70% mixed failures and only 30% adhesive failures, as showed in Table 3 and Fig- ure 5. Discussion The findings of this study reveal significant dif- ferences in mean shear bond strength between 3D printed Co-Cr alloy and heat-cured acrylic resin when subjected to various surface treat- ments, such sandblast (S), metal primer (P), and sandblast +metal primer (SP), compared to the control group. However, there was no sig- nificant difference observed with the acid etch treatment in relaEon to the control.so, the null hypothesis was parEally rejected. The control group (C) and acid-etched group (A) exhibited the lowest shear bond strength values (0.369 ± 0.074 MPa and 0.433 ± 0.081 MPa, respecEvely), as showed in Table 1, indi- caEng poor adhesion at the interface. These re- sults suggest that acid etch or no surface treat- ment is insufficient for achieving clinically ac- ceptable bond strength [26]. The sandblast group (S) showed a substanEal increase in the shear bond strength (5.797 ± 1.011 MPa, as showed in Table 1, reflecEng the posiEve effect of mechanical surface roughen- ing. SandblasEng enhances micromechanical retenEon, which is well-documented as a mean to improve the bond between dental alloys and resin materials [25,27]. However, this group achieved significantly lower SBS values than the primer groups. The primer group (P) and the combinaEon sandblast +metal primer group (SP) achieved the highest shear bond strength values (12.769 ± 2.614 MPa and 15.783 ± 1.193 MPa, respec- Evely) as showed in Table 1. The sandblast + metal primer (SP) group demonstrated staEs- Ecally superior performance compared to all other groups. The synergisEc effect of mechan- ical and chemical surface treatments is sup- ported by literature, which showed that com- bining surface roughening with a suitable pri- mer maximizes both micromechanical and chemical adhesion, resulEng in a more durable and reliable bond [25,27]. The acid-etch surface group had lower shear bond strength values than the other surface treatments. This is maybe due to the presence of oxide film, which plays a vital role in the cor- rosion resistance of the Co-Cr material. Surface oxide film forms spontaneously on alloy sur- faces aWer ambient oxygen exposure, acEng as a barrier to electron flow (resistor) between the electrolyte and the alloy surface, thus pro- tecEng it against corrosion [28]. The oxide layer of the 3D-printed Cr-Co alloys has greater thickness and density than the cast alloys [29]. Furthermore, it is known that a harder material is capable of maintaining a thicker oxide layer more firmly as compared with a soWer material [30], and it was found that the SLM-formed Co- Cr alloy has higher hardness than the cast alloy [14]. The result of this study agrees with previous work [31,32] that concluded that the use of HCL did not improve Etanium–ceramic bond- ing strength values. For aluminum oxide surface treatment group, the present study revealed that sandblasEng significantly enhanced the shear bond strength (SBS) between the Co-Cr alloy and acrylic resin compared to the untreated control and acid- etched groups. This finding aligns with several previous studies that confirmed the positive correlation be- tween surface roughness and bond strength in metal-resin systems [20,33]. These findings are consistent with previous re- ports [20,34,35], which concluded that the Al2O3 abrasive significantly affects the quality of the Co-Cr alloy connection with the acrylic resin. For metal primer surface treatment group, the findings of the current study revealed that the applicaEon of metal primer had a staEsEcally significant posiEve effect on the shear bond strength (SBS) between the Co-Cr alloy and acrylic resin. These results can be attributed to the chemical interaction facilitated by the functional mono- mers in the metal primer, most notably 10- Methacryloyloxydecyl dihydrogen phosphate (10-MDP). This monomer formed strong chem- ical bonds with the oxide layer of Co-Cr alloys, promoting adhesive interaction with the meth- acrylate groups in the resin matrix. This dual af- finity enables the formation of a durable and stable hybrid layer at the alloy–resin interface, which significantly enhances SBS [36-40]. These findings are consistent with previous re- ports [13,20,41], which concluded that the metal primer significantly improved shear bond strength between acrylic resin and differ- ent metal alloys. 3D Printed Co-Cr Alloy Surface Treatments Effect on Shear Strength of Heat Cured Resins Vol 13, No 1 (2025) DOI 10.5195/d3000.2025.1067 http://dentistry3000.pitt.edu 4 CombinaEon sandblast and metal primer sur- face treatment group (SP), the current study revealed that the combinaEon treatment, re- sulted in the highest shear bond strength (15.783 ± 1.193 MPa) among all the experi- mental groups, as showd in Table 1. This re- markable increase in bond strength can be at- tributed to the synergisEc effect of mechanical and chemical surface modificaEons [16]. These findings are consistent with previous re- ports [11,20,42], which concluded that the use of metal primers along with sandblasEng signif- icantly improved the bonding of acrylic den- ture base resin with the Co-Cr alloy. However, this study disagrees with Ghazwan and Nabeel [43], who concluded that using metal primer combined with air abrasion im- proved the shear bond strength, but less than that obtained from specimens treated with metal primer only. This may be due to differ- ences between the two studies, they used Valpast (flexible) denture base material and a different metal primer than that used in this study. LimitaEons of this study included the in vitro design and the dimensions of the test sample used did not represent the actual clinical con- diEons. The difference in the geometry may af- fect the stress distribuEon and hence the shear bond strength and a single brand of metal pri- mer was evaluated and there are addiEonal surface treatments different from those used in this study that were not tested, like fiber la- ser and silica. Conclusion Within the limitaEons of this study, the follow- ing conclusions were drawn: 1. The combinaEon of sandblasEng and metal primer applicaEon demonstrated the highest shear bond strength, indicaEng a synergisEc ef- fect between micromechanical and chemical bonding. In contrast, untreated (control) and acid-etched groups showed low bond strength, emphasizing the limited impact of HCl alone. 2. All samples including: Control, sandblasted, and acid-treated groups exhibited 100% adhe- sive failures, indicating weak bonding, while the primer and SP groups displayed (60%-70%) mixed failures, confirming superior interfacial strength when chemical bonding is involved. Acknowledgements The faculty and staff of the University of Mosul, College of DenEstry have been tremendously helpful throughout this process, and we are very grateful to them. Ethical Approval Before starEng this study, documented ap- proval was obtained from the Research Ethics Commilee (REC) at the College of DenEstry, University of Mosul, Iraq, with a license num- bered (UoM.Dent/25/1011). 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The Journal of Prosthe9c Den9stry, 2025. 134(1): p. 114-121. 43. Ghazwan, A. and A. Nabeel, Effect of different metal surface treatments and thermocycling on shear bond strength and microleakage of the flexible acrylic at Co/Cr interface. Journal Of Baghdad College Of Den9stry, 2011. 23: p. 10-4. . Table 1. DescripEve staEsEcs of the shear bond strength test for the studied groups. Groups N Mean (MPa) Standard Devia- Eon Minimum Maximum C 10 0.369 0.07415 0.27 0.46 A 10 0.433 0.08111 0.32 0.59 S 10 5.797 1.01101 4.43 7.43 p 10 12.769 2.61366 10.13 17.13 SP 10 15.783 1.19342 13.72 17.36 N: Total number of measurements. C: Control; A: Acid; S: Sandblast; P: Metal Primer; SP: Sandblast + Metal Primer. Table 2. One-way ANOVA test for the shear bond strength test. Sum of Squares Degrees of Freedom Mean Square F p-value Between Groups 1989.607 4 497.402 267.718 0.000 Within Groups 83.607 45 1.858 Total 2073.214 49 3D Printed Co-Cr Alloy Surface Treatments Effect on Shear Strength of Heat Cured Resins Vol 13, No 1 (2025) DOI 10.5195/d3000.2025.1067 http://dentistry3000.pitt.edu 6 Table 3. DistribuEon of failure modes between metal and denture base resin bonding following the shear bond strength test. Group Adhesive Cohesive Mixed C 10 0 0 A 10 0 0 S 10 0 0 P 4 0 6 S P 3 0 7 C: Control; A: Acid; S: Sandblast; P: Metal Primer; SP: Sandblast + Metal Primer. Figure 1. Diagram of the metal mold for wax applicaEon [11]. 3D Printed Co-Cr Alloy Surface Treatments Effect on Shear Strength of Heat Cured Resins Vol 13, No 1 (2025) DOI 10.5195/d3000.2025.1067 http://dentistry3000.pitt.edu 7 Figure 2. Final shear bond strength specimen shape aWer molten wax applicaEon. Figure 3. Specimens mounted in the universal tesEng machine. 3D Printed Co-Cr Alloy Surface Treatments Effect on Shear Strength of Heat Cured Resins Vol 13, No 1 (2025) DOI 10.5195/d3000.2025.1067 http://dentistry3000.pitt.edu 8 Figure 4. Duncan’s mulEple range test for the shear bond strength test for the studied groups. Figure 5. Types of failures: (A) adhesive failure. (B) mixed-type failure.