Table III. Mechanical properties of transmissive optical materials

Material	Symbol	Strength Fa	Fracture Toughness	Hardness (Knoop)	Density D	Comments
		MPa	MPa·m½	kgf/mm2	g/cm3
Sodium chloride	NaCl	2.4 (9.5)	?	18	2.16	cleaves
Potassium chloride	KCl	2.2 (11)	?	17	1.98	least hygroscopic salt, cleaves
Potassium bromide	KBr	1.1	?	7	2.75	cleaves
Cesium bromide	CsBr	8.3	?	20	4.44	shock resistant, does not cleave
Cesium iodide	CsI	2.7 (5.6)	?	20	4.53	shock resistant, deforms (cold flow)
Lithium fluoride	LiF	11	?	99	2.64	sensitive to thermal shock, cleaves
Magnesium fluoride	MgF2	49	1.0	415 (575)	3.18	rugged, cleaves
Calcium fluoride	CaF2	37 (54)	0.5	158	3.18	hard, fairly tough
Strontium fluoride	SrF2	~35	?	154	4.28
Barium fluoride	BaF2	27	?	82	4.88	sensitive to mechanical shock
Silver chloride	AgCl	26 (<1)	?	10	5.56	deforms (cold flow)
Silver bromide	AgBr	~30 (1.1)	?	7	6.47	deforms (cold flow)
Thallium bromochloride	KRS-6	21	?	39	7.19	deforms (cold flow), dust very toxic
Thallium bromoiodide	KRS-5	26	?	40	7.37	deforms (cold flow), dust very toxic
Magnesium oxide	MgO	27	?	692	3.58	tough
Sapphire	a Al2O3	448 (689)	3.0	1370	3.98	very hard, tough
Crystal quartz	a SiO2	~41	?	820	2.65	brittle, inert, cleaves
Zirconia	ZrO2	?	2.0	~1160	5.85
Zinc sulfide	ZnS	60 (103)	0.5 (0.8)	160 (250)	4.09	tough, dust allergenic
Zinc selenide	ZnSe	55	0.33	120	5.27	fairly tough, dust allergenic
Cadmium sulfide	CdS	28	?	108	4.82	birefringent, dust toxic
Cadmium selenide	CdSe	?	?	~70	5.67	brittle, birefringent, dust toxic
Cadmium telluride	CdTe	~6	?	56	5.85	very brittle, dust toxic, visually opaque
Diamond	C	370 (8700)	2.0	8820	3.52	very hard & inert, cleaves
Silicon	Si	~62	0.95	1150	2.33	very hard & inert, visually opaque
Germanium	Ge	~49	0.66	780	5.33	very brittle, visually opaque
Gallium arsenide	GaAs	~50	?	721	5.32	dust toxic
Silica glasses	vSiO2	~35	0.8	461	2.20
Borosilicate glasses	BSCG	~35	?	~500	2.35
Fluorozirconate glass	HMFG	~25	?	225	4.33	sensitive to thermal shock
Arsenic trisulfide glass	As2S3	~8	?	109	3.20	brittle, dust toxic
AMTIR-1 glass	AMTIR	~18	?	170	4.40	brittle, visually opaque, dust toxic
GASIR1 glass	GASIR1	~17	?	?	4.40	visually opaque, dust toxic, moldable
GASIR2 glass	GASIR2	~18	?	172	4.70	visually opaque, dust toxic, moldable
Poly-ethylene	HDPE	27	?	~9 (R60)	0.95	LDPE deforms, hardness only ~4 (R15)
Poly-methylpentene	TPX	~24	?	~8 (R60)	0.83	fairly rigid
Polyester	PET	~98	?	~13 (R110)	1.37	strong, e.g. stretched-film windows
Poly-tetrafluoroethylene	PTFE	~22	?	~7 (R58)	2.24	deforms (cold flow), visually opaque
Fluorinated EPC	FEPC	~18	?	~5 (R25)	2.15	deforms (cold flow)

Strength (Fa) is defined as the elastic limit or flexural apparent elastic limit.
Crystalline strength is often limited by brittleness & cleavage; higher effective strength can often be achieved by careful orientation & mounting.
Parenthetical Fa values for CaF2, NaCl, KCl, & ZnS are for polycrystalline materials.
Parenthetical Fa value for CsI is for work-hardened material.
Parenthetical Fa values for AgCl & AgBr are empirical "strengths" for long-term pressure applications where cold flow is the determining factor.
The low Fa value for diamond reflects its brittleness & easy cleavage; its parenthetical Fa value is achieved by careful orientation & mounting.
Fracture toughness measures the difficulty of propagating an incipient fracture.
Fracture toughness value for ZnS is for the crystalline material; the parenthetical value is for CVD material.
Parenthetical hardness values for MgF2 and ZnS are for hot-pressed materials (i.e., Irtran).
Knoop hardness values for plastic resins are rough estimates. Measured hardness values for plastic resins are in Rockwell "R" units.
These data were collected from a variety of sources, so this table probably contains errors. Use cautiously, and empirically verify critical values.
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