Last updated: August 14, 2026
Silicon carbide for refractory applications is available in different SiC content grades, and the highest SiC percentage is not automatically the best choice for every formulation. SiC content, free carbon, oxide impurities, particle size and the operating conditions of the refractory product should be considered together.
This guide compares common refractory silicon carbide grades such as SiC 85, SiC 75, SiC 70 and SiC 65, explains what changes as SiC content decreases, and shows what buyers should check before selecting a grade.
Quick Answer
For refractory applications, silicon carbide grade should not be selected by SiC percentage alone. Higher-SiC grades contain a larger proportion of silicon carbide, while lower grades generally contain more secondary phases and impurities. However, actual refractory performance also depends on free carbon, oxide content, particle-size distribution, binder system and service conditions.
SiC 85 may be preferred where a higher SiC contribution is required, while SiC 75, 70 or 65 may be practical in formulations where cost, aggregate structure and impurity tolerance allow a lower-grade material.
The final decision should be based on the complete chemical specification and the refractory formulation rather than the grade name alone.
Silicon Carbide Grades at a Glance
The grade name normally indicates the approximate minimum SiC level used to classify the material. It does not fully describe the impurity profile or physical characteristics.
| Grade | SiC Level | Impurity Control | General Selection Direction |
|---|---|---|---|
| SiC 85 | Higher SiC content | Check F.C., Fe₂O₃, SiO₂ and other limits from actual specification | Refractory formulations requiring a higher proportion of SiC |
| SiC 75 | Medium-high SiC content | Impurity profile should be checked together with SiC content | General refractory formulations balancing SiC content and raw-material cost |
| SiC 70 | Medium SiC content | Higher secondary-phase content may be present depending on production route | Cost-sensitive refractory formulations where lower SiC content is acceptable |
| SiC 65 | Lower SiC content | Complete chemistry becomes especially important when comparing offers | Formulations where SiC contribution is required but very high purity is not necessary |
These grade names are not complete purchasing specifications. Two materials sold as "SiC 75," for example, may have different free carbon, iron oxide, silica, moisture and particle-size limits.
What Changes as SiC Content Increases?
Increasing the SiC percentage means that a larger proportion of the raw material consists of silicon carbide. This can affect how much SiC the material contributes to a refractory formulation, but purity should not be treated as a direct guarantee of final refractory performance.
Higher Effective SiC Input
A higher-grade material supplies more SiC per unit mass. This may be useful when the refractory formulation is designed around a relatively high silicon carbide fraction.
For the same target SiC input, a lower-grade material generally requires a greater total raw-material addition than a higher-grade material.
Different Secondary-Phase Content
The portion of the material that is not SiC may include free carbon, silica, iron-bearing oxides and other constituents depending on the raw material and production process.
This means that changing from SiC 85 to SiC 70 is not simply a reduction in the amount of SiC. It can also change the total impurity input into the refractory formulation.
Different Raw-Material Cost
Higher-purity material normally requires greater processing and raw-material control, so the purchasing cost may be higher.
However, comparing only the price per ton can also be misleading. Buyers should consider the effective SiC input, impurity limits and whether the lower-cost grade can meet the formulation requirements.
SiC 85 vs 75 vs 70 vs 65: How to Choose
The most suitable grade depends on what the SiC is expected to do in the refractory formulation and how much impurity input the formulation can tolerate.
When to Consider SiC 85
SiC 85 is a practical direction when the formulation requires a relatively high SiC contribution and the buyer wants to reduce the proportion of non-SiC components introduced with the raw material.
It may be considered for more demanding refractory formulations, but the actual suitability still depends on the complete chemical specification and particle-size requirement.
Do not assume that every SiC 85 product has the same free carbon or Fe₂O₃ level. These values should be confirmed from the supplier specification or batch COA.
When to Consider SiC 75
SiC 75 can provide a balance between SiC content and raw-material cost when the formulation does not require a higher-purity grade.
For this grade, buyers should pay particular attention to how the remaining non-SiC portion is distributed between free carbon, oxide phases and other impurities.
SiC 75 should therefore be selected from the complete chemistry rather than from the 75% figure alone.
When to Consider SiC 70
SiC 70 may be suitable for cost-controlled refractory formulations where a moderate SiC contribution is required and the formulation has sufficient tolerance for secondary components.
The buyer should confirm whether the actual impurity profile is compatible with the binder system, aggregate combination and service environment.
When to Consider SiC 65
SiC 65 is a lower-SiC grade and may be used where high purity is not the primary requirement.
Because a larger portion of the product is made up of components other than SiC, the detailed analysis becomes increasingly important. F.C., oxide content and particle-size consistency should be checked before treating two SiC 65 offers as equivalent.
Why Do Impurities Matter as Much as SiC Content?
The SiC percentage tells you how much silicon carbide is present, but it does not explain what makes up the rest of the material.
For refractory purchasing, the following parameters should be checked together.
Free Carbon (F.C.)
Free carbon is carbon that is not chemically combined as SiC.
Its effect depends on the refractory formulation and application. A certain free-carbon level may be acceptable or even compatible with one formulation but unsuitable for another.
For this reason, F.C. should be treated as a separate specification rather than inferred from the SiC grade.
Iron Oxide
Fe₂O₃ or other reported iron-bearing components can be important when the refractory formulation has strict impurity requirements.
Two products with the same SiC percentage can have different iron levels, so the Fe specification should be checked independently.
Silica and Other Oxides
SiO₂, CaO and other oxide constituents may be reported depending on the grade and supplier specification.
Their significance depends on the refractory matrix, binder system and operating environment. They should not be assumed to have the same limits across all SiC grades.
Moisture
Moisture is particularly relevant to storage, handling and batching consistency.
If moisture is controlled in your purchasing specification, confirm the actual maximum limit and how the material is packed and stored before shipment.
Does Higher SiC Always Mean Better Refractory Performance?
No. A higher SiC percentage does not automatically produce a better refractory product.
Final refractory performance is determined by the complete formulation and service conditions, including:
- total SiC content in the formulation,
- aggregate and fine-powder particle-size distribution,
- binder type and matrix composition,
- porosity and density of the finished refractory,
- firing or curing conditions,
- working temperature,
- slag, metal or gas contact,
- and thermal cycling.
A correctly designed formulation using SiC 75 may therefore be more appropriate for a particular application than simply replacing the material with SiC 85 without adjusting the rest of the formulation.
The purpose of grade selection is to meet the required formulation chemistry and performance target at a reasonable raw-material cost.
How Does Particle Size Affect SiC Selection?
Particle size is a separate purchasing parameter from chemical grade.
SiC 85 does not correspond to one fixed particle size, and SiC 75 does not automatically require a different size. The same chemical grade can be supplied in different fractions depending on how it is used in the refractory formulation.
| Material Function | Particle-Size Direction | What to Confirm |
|---|---|---|
| Coarse Aggregate | Coarser fraction | Top size, oversize and fines |
| Intermediate Aggregate | Medium fraction | Distribution and screening consistency |
| Fine Addition | Fine grains or powder | Mesh or micron requirement |
| Graded Refractory Mix | Combination of fractions | Complete particle-size distribution |
When requesting a quotation, it is better to specify the required fraction or particle-size distribution directly instead of ordering only by "SiC 75" or "SiC 85."
How Should Buyers Compare SiC Grade Offers?
Price per ton alone is not enough to compare two refractory silicon carbide offers.
A practical comparison should place the following information side by side:
| Parameter | Why It Matters |
|---|---|
| SiC Minimum | Defines the main grade and effective SiC input |
| Free Carbon | Shows carbon present outside the SiC phase |
| Fe₂O₃ / Iron | Important where iron-bearing impurities are restricted |
| SiO₂ / Other Oxides | Helps evaluate the non-SiC portion of the material |
| Moisture | Relevant to handling and batching consistency |
| Particle Size | Must match the refractory formulation |
| Batch COA | Confirms actual delivered chemistry |
| Packing | Affects handling, storage and transport |
If two suppliers quote the same grade name but provide different impurity limits, they should not automatically be treated as equivalent products.
What Should Be Included in an SiC Purchasing Specification?
A clear purchasing specification reduces ambiguity between the refractory producer and the SiC supplier.
Instead of requesting only "SiC 75" or "SiC 85," include:
- SiC minimum
- Free carbon limit or acceptable range
- Fe₂O₃ or iron limit
- SiO₂ and other required oxide limits
- Moisture limit
- Required particle size or distribution
- Packaging requirement
- Quantity
- COA or inspection requirement
- Refractory application
If you already have an existing raw-material specification, sending the complete chemistry and particle-size limits is more useful than requesting a quotation based only on the SiC grade name.
Particle Size, Packing and Quality Documents
Refractory silicon carbide can be supplied in different particle-size fractions and packing formats according to the order specification.
Before placing an order, confirm:
- the required grain-size range or mesh,
- limits for fines and oversize where necessary,
- bag size and outer packing,
- batch identification,
- chemical analysis report,
- and particle-size report where required.
For available grades and current supply specifications, see our silicon carbide for refractory applications.
Which Standards Apply to Refractory Silicon Carbide?
Standards should be used according to their actual scope rather than added to a specification simply because they mention silicon carbide.
For SiC-containing refractory raw materials and refractory products, the ISO 21068 series provides methods for chemical analysis of silicon-carbide-containing materials.
Depending on the applicable part and analysis requirement, the series covers parameters such as silicon carbide, total carbon, free carbon, free silicon, silica and other metallic or oxide constituents.
These test methods do not define SiC 65, 70, 75 or 85 as universal commercial grades. The actual grade limits still need to be agreed between the supplier and buyer or defined by the applicable purchasing specification.
Standards intended for abrasive-grade silicon carbide should not automatically be presented as refractory-grade specifications unless the supplied material is actually tested and purchased under that standard.
How to Select the Right Silicon Carbide Grade
Step 1: Start with the Refractory Formulation
Determine how much SiC the formulation requires and what function the SiC performs in the material.
Step 2: Define the Minimum SiC Content
Choose the grade range based on the required SiC contribution rather than automatically selecting the highest available purity.
Step 3: Check the Impurity Limits
Compare F.C., Fe₂O₃, SiO₂, moisture and any other controlled components.
Step 4: Define Particle Size
Specify whether the material will be used as coarse aggregate, intermediate grain, fine addition or powder.
Step 5: Compare the Real Material Cost
Compare not only price per ton but also SiC input, impurity profile and whether the material fits the existing formulation without unnecessary adjustment.
Step 6: Confirm with Batch Data
Use the supplier specification, COA and production trial data where necessary to confirm that the selected grade performs consistently in the refractory formulation.
Key Takeaways
- SiC 85, 75, 70 and 65 should not be compared by SiC percentage alone.
- Free carbon, Fe₂O₃, SiO₂, moisture and particle size can differ even when two products use the same grade name.
- Higher SiC content does not automatically guarantee better refractory performance because the complete formulation and service conditions control the final result.
- Particle size is independent of chemical grade and should be specified according to the refractory formulation.
- The most useful purchase specification combines chemistry, particle size, packing and required quality documents.
FAQ About Silicon Carbide Grades
Is SiC 85 always better than SiC 75 for refractories?
No. SiC 85 provides a higher proportion of silicon carbide, but the correct grade depends on the refractory formulation, impurity tolerance, particle-size requirement, service conditions and raw-material cost. If SiC 75 already meets the formulation requirements, using SiC 85 may not provide a practical benefit.
Can two SiC 75 products have different specifications?
Yes. The grade name mainly describes SiC content. Free carbon, Fe₂O₃, SiO₂, moisture, particle-size distribution and other limits can differ between products and suppliers. The complete specification should be compared before purchase.
Does silicon carbide grade determine particle size?
No. Chemical grade and particle size are separate parameters. The same SiC grade can be supplied as different grain fractions or powders depending on the refractory formulation and processing requirement.
What information should I provide when requesting refractory SiC?
Provide the required SiC minimum, impurity limits, particle size, quantity, packing requirement and refractory application. If you already have an existing specification or COA, sending it directly allows different offers to be compared on the same basis.
Need to Compare Refractory SiC Grades?
If you are choosing between SiC 85, SiC 75, SiC 70 or another refractory silicon carbide grade, send us your required SiC content, impurity limits, particle size and application.
If you already have a raw-material specification or previous COA, we can compare the chemistry on the same basis instead of selecting a material from the SiC percentage alone.
