Strengthening

Tempering, thermal strengthening and chemical strengthening are the three primary methods for increasing strength and heat resistance of glass. Through partnerships, CiDRA® Precision Services, LLC offers all three of these processes for applications requiring high strength glass components.  

Tempering:   Tempering uses heat treatment processes to create compressive residual stresses on the surface of glass as a means of increasing strength and thermal shock resistances without impacting most other physical properties.   To be compliant with ASTM-C 1048, the residual surface compressive stresses must exceed 10,000psi (69 MPa) or edge compressive stresses must exceed 9,700psi (67 MPa).    Tempered glass will break if the compressive layer is penetrated by a chip or crack, but the breakage is characterized by many small pieces of glass.  The residual stresses generated by tempering can also cause warping in sheet glass and as a result, is not usually effective for sheet glass less than 3mm in thickness.   Tempered glass can operate at temperatures up to 243°C without degradation but will shatter at temperatures greater that 316°C .

Heat Strengthening:   The processes used in heat strengthening are very similar to those used in tempering, with only the end state being different.  Heat strengthening creates compressive residual stresses between 3,500psi (24 MPa) and 10,000psi (69 MPa), or edge stresses between 5,500psi (38 MPa) and 9,700psi (67 MPa).  Unlike tempered glass, heat strengthened glass generally fractures in a manner similar to annealed glass.   The residual stresses can cause warping in sheet glass, and is not usually effective for sheet glass less than 3mm in thickness. 

Chemical Strengthening:   Chemical strengthening uses molten salt baths to promote an ion exchange that produces compressive stresses at the surface of glasses with high sodium oxide content, such as soda lime and borofloat glasses.  Compressive stresses as high as 43,500psi (300 MPa)  in soda lime glass and 14,500psi (100 MPa)  in borofloat glass are produced in a ~20um layer at the glass surface.  Chemical strengthening is often used in applications not well suited for tempering and  is especially useful for glass sheet less than 3mm thick, where flatness is a critical characteristic.

Comparison of Propriety Changes Due to Strengthening
Property Change Due to Tempering Change Due to Heat Strengthening Change Due to Chemical Strengthening
Impact Resistance(*) 5x to 6x   4x to 5x
Bending Strength(*) 4x to 5x 2x 3x
Resistance to Temperature(*) 4x 2.5x 1.8x to 2.5x
Vickers Hardness (*)     1.14x
Maximum Temperature 243°C 230°C 300°C
Compressive Stress
at Surface		 >69MPa 24MPa – 69MPa 100MPa – 300MPa

(*) – Relative increase over annealed glass.  5x means 5 times larger.

Machining Strengthened Glass:  Glass cannot be machined after tempering or heat strengthening.  Features machined prior to tempering or heat strengthening must be smooth, crack free and chip free.   Holes and notches must be properly located to avoid breakage during strengthening. 

Although not advisable, chemical strengthened glass can withstand certain machining processes after strengthening.  However, even if the glass survives the machining, it will lose its added strength in a region encompassing approximately one inch from the machined feature.

Machining Rules of Thumb for Strengthened Glass
Property General Rule
Minimum Hole Diameter glass thickness
Distance of Hole Perimeter from the Edge of the Glass 5 x glass thickness
Distance Between Hole Perimeters 5 x glass thickness
Distance from Corners to Hole Perimeters 5 x glass thickness
Minimum Fillet Radius glass thickness