I. i%omwchun~s Vd 23. No. 9. pi Pmld In Glul nmaln
912-925. 1990.
EFFECTS OF SURFACE ROUGHNESS ON THE COEFFICIENTS OF FRICTION IN MODEL ORTHODONTIC SYSTEMS R. P. Kusv*t
and J. Q. WHITLEY
Dental Research Centcr/Dept of Orthodontics and Biomedical Engineering, University of North Carolina, Chapel Hill, NC 27599-7455, U.S.A. Abstract-Orthodontists. like others (Engel. P. A. (1976) Impclct Wear ojMateri&. Elsevier Scientific. New York.). often equate the smoothness of surfaces with the absence of friction. To investigate whether the surface roughnessesof opposing materials influence the coefficients of friction and ultimately the movement of teeth, arch wires were slid between contact fiats to simulate orthodontic arch wire-bracket appliances. From laser specular reflectance measurements, the RMS surface roughnessesof thesearch wires varied from 0.04 pmlfor stainless steel to 0.23 gm for nickel titanium. Using the same technique, the roughnessesof the contact flats varied from 0.03 pm for the I pm lapped stainless steel, to 0.26 pm for the as-received alumina. After each of the arch wire-contact flat couples was placed in a friction tester. fifteen normal forces were systematically applied at 34°C. From plots of the static and kinetic frictional forces vs the normal forces, dry coefficiehts of friction were obtained that were greater than those reported in the dental literature. The allstainless steel couples had lower kinetic coefficients (0.120-0.148) than the stainless steel-polycrystalline alumina couple (0.187). When pressedagainst the various Rats. the beta-titanium arch wire (RMS = 0. I4 pm) had the highest coefficientsof friction (0.441-0.658). although the nickel titanium arch wire was the roughest (RMS =0.23 pm). Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) verified that mass transfer of the beta-titanium arch wire occurred by adhesion onto the stainless steel flats or by abrasion from the sharply faceted polycrystalline alumina flats.
INTRODUCTION Often the orthodontist prescribes a treatment plan that includes siliding mechanics to move teeth. One such occasion, albeit somewhat controversial according to Burstone (1982) and Profftt (1986) involves the space closure of premolar extraction sites over a period of six months. A more generally accepted occasion involves the sliding of irregular teeth, such as crooked incisors, along the arch wire over a period of less than three months. Notwithstanding, the sliding processrequires the selection of materials from among an extensive inventory of metal, ceramic, polymer, and composite materials. Each of the arch wire-bracket combinations has a unique set of physical properties that may make it perform differently from others. Several investigators have focused on the performance of the more commonly used arch wirebracket combinations and have found that frictional forces increases with wire size (Andreasen and Quevedo, 197OtRilley et al., 1979; Frank and Nikolai, 1980). bracket angle (Andreasen and Quevedo, 1970; Frank and Nikolai, 1980; Nicolls. 1968). or as ligation forces increase (Frank and Nikolai, 1980; Nicolls, 1968). With one exception (Andreasen and Quevedo, 1970). frictional forces appeared to increase with bracket width (Frank and Nikolai, 1980; Nicolls, 1968). No con$istent picture emerges, however, with regard to the effects of either wire alloy (Garner et al.. Received in fin@/ firm 9 February 1990. *Author IO whom correspondence should be addressed. t Present addmss: Bldg 210-H. Room 313. University of North Carolina, Chapel Hill, NC 27X9-7455, U.S.A.
1986; Stannard ef al.. 1986) or fluids, especially saliva (Andreasen and Quevedo. 1970, Rilley et cl., 1979; Nicolls, 1968; Stannard ef al.. 1986; Koran er al., 1972). Surface roughness, hardness, stiffness, yield strength, and relative velocity are sometimes acknowledged as potential factors (Frank and Nikolai, 1980; Stannard et al., 1986). but nothing has been done to establish the extent of their influence. To understand more fully the critical parameters of sliding mechanics, a series of studies have been initiated. The first study considers whether surfaa roughness etfects the dry coefficient of friction. This coefficient inevitably becomes important in the oral environment whenever the boundary layer of saliva is squeezed out from between the arch wire-bracket contact. Using sixteen arch wire-contact flat combinations, model orthodontic systems were constructed. By surface roughness and coefficients of friction measurements, the hypothesis that a smoother surface correlates with a lower coefficient of friction was tested. MATERIALS AND METHODS Four arch wire materials were selected that represented AlSl type 302 or 304 stainless steel, cobalt-chromium, nickel titanium, and beta-titanium alloys (cf. Table 1). All were 0.018” x 0.025” rectangular wires except for the TMAr” product, which was an 0.017” x 0.025” arch wire. The as-received appearana of these wires (without drawing lubricants, which were ultrasonically removed) show die scratches running in the drawing/rolling direction (Fig. 1).
913
914
R. P. KUSY and J. Q.
WHtTLEY
Table 1. Materials evaluated Arch wires Gcnbral class (nominal wt%) Stainless steel (71 Fee.l8Cr. 8 Ni,
912-925. 1990.
EFFECTS OF SURFACE ROUGHNESS ON THE COEFFICIENTS OF FRICTION IN MODEL ORTHODONTIC SYSTEMS R. P. Kusv*t
and J. Q. WHITLEY
Dental Research Centcr/Dept of Orthodontics and Biomedical Engineering, University of North Carolina, Chapel Hill, NC 27599-7455, U.S.A. Abstract-Orthodontists. like others (Engel. P. A. (1976) Impclct Wear ojMateri&. Elsevier Scientific. New York.). often equate the smoothness of surfaces with the absence of friction. To investigate whether the surface roughnessesof opposing materials influence the coefficients of friction and ultimately the movement of teeth, arch wires were slid between contact fiats to simulate orthodontic arch wire-bracket appliances. From laser specular reflectance measurements, the RMS surface roughnessesof thesearch wires varied from 0.04 pmlfor stainless steel to 0.23 gm for nickel titanium. Using the same technique, the roughnessesof the contact flats varied from 0.03 pm for the I pm lapped stainless steel, to 0.26 pm for the as-received alumina. After each of the arch wire-contact flat couples was placed in a friction tester. fifteen normal forces were systematically applied at 34°C. From plots of the static and kinetic frictional forces vs the normal forces, dry coefficiehts of friction were obtained that were greater than those reported in the dental literature. The allstainless steel couples had lower kinetic coefficients (0.120-0.148) than the stainless steel-polycrystalline alumina couple (0.187). When pressedagainst the various Rats. the beta-titanium arch wire (RMS = 0. I4 pm) had the highest coefficientsof friction (0.441-0.658). although the nickel titanium arch wire was the roughest (RMS =0.23 pm). Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) verified that mass transfer of the beta-titanium arch wire occurred by adhesion onto the stainless steel flats or by abrasion from the sharply faceted polycrystalline alumina flats.
INTRODUCTION Often the orthodontist prescribes a treatment plan that includes siliding mechanics to move teeth. One such occasion, albeit somewhat controversial according to Burstone (1982) and Profftt (1986) involves the space closure of premolar extraction sites over a period of six months. A more generally accepted occasion involves the sliding of irregular teeth, such as crooked incisors, along the arch wire over a period of less than three months. Notwithstanding, the sliding processrequires the selection of materials from among an extensive inventory of metal, ceramic, polymer, and composite materials. Each of the arch wire-bracket combinations has a unique set of physical properties that may make it perform differently from others. Several investigators have focused on the performance of the more commonly used arch wirebracket combinations and have found that frictional forces increases with wire size (Andreasen and Quevedo, 197OtRilley et al., 1979; Frank and Nikolai, 1980). bracket angle (Andreasen and Quevedo, 1970; Frank and Nikolai, 1980; Nicolls. 1968). or as ligation forces increase (Frank and Nikolai, 1980; Nicolls, 1968). With one exception (Andreasen and Quevedo, 1970). frictional forces appeared to increase with bracket width (Frank and Nikolai, 1980; Nicolls, 1968). No con$istent picture emerges, however, with regard to the effects of either wire alloy (Garner et al.. Received in fin@/ firm 9 February 1990. *Author IO whom correspondence should be addressed. t Present addmss: Bldg 210-H. Room 313. University of North Carolina, Chapel Hill, NC 27X9-7455, U.S.A.
1986; Stannard ef al.. 1986) or fluids, especially saliva (Andreasen and Quevedo. 1970, Rilley et cl., 1979; Nicolls, 1968; Stannard ef al.. 1986; Koran er al., 1972). Surface roughness, hardness, stiffness, yield strength, and relative velocity are sometimes acknowledged as potential factors (Frank and Nikolai, 1980; Stannard et al., 1986). but nothing has been done to establish the extent of their influence. To understand more fully the critical parameters of sliding mechanics, a series of studies have been initiated. The first study considers whether surfaa roughness etfects the dry coefficient of friction. This coefficient inevitably becomes important in the oral environment whenever the boundary layer of saliva is squeezed out from between the arch wire-bracket contact. Using sixteen arch wire-contact flat combinations, model orthodontic systems were constructed. By surface roughness and coefficients of friction measurements, the hypothesis that a smoother surface correlates with a lower coefficient of friction was tested. MATERIALS AND METHODS Four arch wire materials were selected that represented AlSl type 302 or 304 stainless steel, cobalt-chromium, nickel titanium, and beta-titanium alloys (cf. Table 1). All were 0.018” x 0.025” rectangular wires except for the TMAr” product, which was an 0.017” x 0.025” arch wire. The as-received appearana of these wires (without drawing lubricants, which were ultrasonically removed) show die scratches running in the drawing/rolling direction (Fig. 1).
913
914
R. P. KUSY and J. Q.
WHtTLEY
Table 1. Materials evaluated Arch wires Gcnbral class (nominal wt%) Stainless steel (71 Fee.l8Cr. 8 Ni,
